An underground film is a film that is out of the mainstream either in its style, genre or financing.
Notable examples include: John Waters' Pink Flamingos , David Lynch's Eraserhead , Andy Warhol's Blue Movie , Rosa von Praunheim's Tally Brown, New York , Frank Henenlotter's Basket Case, Nikos Nikolaidis' Singapore Sling, Rinse Dreams' Café Flesh , and Jörg Buttgereit's Nekromantik .
The first printed use of the term "underground film" occurs in a 1957 essay by American film critic Manny Farber, "Underground Films." Farber uses it to refer to the work of directors who "played an anti-art role in Hollywood." He contrasts "such soldier-cowboy-gangster directors as Raoul Walsh, Howard Hawks, William Wellman," and others with the "less talented De Sicas and Zinnemanns [who] continue to fascinate the critics."  However, as in "Underground Press", the term developed as a metaphorical reference to a clandestine and subversive culture beneath the legitimate and official media.
In the late 1950s, "underground film" began to be used to describe early independent film makers operating first in San Francisco, California and New York City, New York, and soon in other cities around the world as well, including the London Film-Makers' Co-op in Britain and Ubu Films in Sydney, Australia. The movement was typified by more experimental filmmakers working at the time like Shirley Clarke,  Stan Brakhage, Harry Everett Smith, Maya Deren, Andy Warhol,  Kenneth Anger,  Jonas Mekas, Ken Jacobs, Ron Rice, Jack Smith, George and Mike Kuchar, and Bruce Conner.
By the late 1960s, the movement represented by these filmmakers had matured, and some began to distance themselves from the countercultural, psychedelic connotations of the word, preferring terms like avant-garde or experimental to describe their work.
Having been embraced most emphatically by Nick Zedd and the other filmmakers associated with the New York-based Cinema of Transgression and No Wave Cinema of the late 1970s to early 1990s, the term would still be used to refer to the more countercultural fringe of independent cinema.
In the early 1990s, the legacy of the Cinema of Transgression carried over into a new generation, who would equate "underground cinema" with transgressive art, ultra-low-budget filmmaking created in defiance of both the commercialized versions of independent film offered by newly wealthy distributors like Miramax and New Line, as well as the institutionalized experimental film canonized at major museums. This spirit defined the early years of underground film festivals (like the New York Underground Film Festival, Chicago Underground Film Festival, Boston Underground Film Festival, Sydney Underground Film Festival,  Hamilton Underground Film Festival, Toronto's Images Festival, and others), zines like Film Threat, as well as the works of filmmakers like Craig Baldwin, Jon Moritsugu, Carlos Atanes, Johnny Terris, Sarah Jacobson, and Bruce La Bruce. In London the Underground resurgence emerged as a movement of Underground cinema clubs which included the radical open access group the Exploding Cinema.
By the late 1990s and early 2000s, the term had become blurred again, as the work at underground festivals began to blend with more formal experimentation, and the divisions that had been stark ones less than a decade earlier now seemed much less so. If the term is used at all, it connotes a form of very low budget independent filmmaking, with perhaps transgressive content, or a lo-fi analog to post-punk music and cultures.
A recent development in underground filmmaking can be observed through the Lower East Side based film production company ASS Studios. Founded in 2011 by writer Reverend Jen and filmmaker Courtney Fathom Sell, the group avoided most modern methods of production, choosing to shoot all of their work on an outdated Hi 8 format and usually with no-budget. Utilizing many New York based performers, their work generally contained camp elements and taboo themes. These films were commonly screened at venues & bars in and around New York City.  
The term "underground film" is occasionally used as a synonym for cult film (as in the case of films like Eating Raoul ).  Though there are important distinctions between the two, a significant overlap between these categories is undeniable. The films of Kenneth Anger, for example, could arguably be described as underground  while a studio film like Heathers (New World Pictures) may have a cult following, but could not be accurately described as an underground film. 
Film critic Pauline Kael called most underground cinema "a creature of publicity and mutual congratulations on artistry". 
List is incomplete. 
An independent film, independent movie, indie film, or indie movie is a feature film or short film that is produced outside the major film studio system, in addition to being produced and distributed by independent entertainment companies. Independent films are sometimes distinguishable by their content and style and the way in which the filmmakers' personal artistic vision is realized. Usually, but not always, independent films are made with considerably lower budgets than major studio films.
Scorpio Rising is a 1963 American experimental short film shot, edited, co-written and directed by Kenneth Anger, and starring Bruce Byron as Scorpio. Loosely structured around a prominent soundtrack of 1960s pop music, it follows a group of bikers preparing for a night out.
Experimental film or avant-garde cinema is a mode of filmmaking that rigorously re-evaluates cinematic conventions and explores non-narrative forms or alternatives to traditional narratives or methods of working. Many experimental films, particularly early ones, relate to arts in other disciplines: painting, dance, literature and poetry, or arise from research and development of new technical resources.
BolexInternational S. A. is a Swiss manufacturer of motion picture cameras based in Yverdon located in Canton of Vaud, the most notable products of which are in the 16 mm and Super 16 mm formats. Originally Bol, the company was founded in 1925 by Charles Haccius and Jacques Bogopolsky, the company's name having been derived from Bogopolsky's name. In 1923 he presented the Cinégraphe Bol at the Geneva fair, a reversible apparatus for taking, printing, and projecting pictures on 35 mm. film. He later designed a camera for Alpa of Ballaigues in the late 1930s.
Jack Sargeant is a British writer specializing in cult film, underground film, and independent film, as well as subcultures, true crime, and other aspects of the unusual. In addition he is a film programmer, curator, academic and photographer. He has appeared in underground films and performances. He currently lives in Australia.
No wave cinema was an underground filmmaking movement that flourished on the Lower East Side of New York City from about 1976 to 1985. Associated with the artists’ group Collaborative Projects, no wave cinema was a stripped-down style of guerrilla filmmaking that emphasized dark edgy mood and unrehearsed immediacy above many other artistic concerns – similar to the parallel no wave music movement in its raw and rapid style.
Emanuel Farber was an American painter, film critic and writer. Often described as "iconoclastic", Farber developed a distinctive prose style and set of theoretical stances which have had a large influence on later generations of film critics and influence on underground culture. Susan Sontag considered him to be "the liveliest, smartest, most original film critic this country has ever produced."
Anthology Film Archives is an international center for the preservation, study, and exhibition of film and video, with a particular focus on independent, experimental, and avant-garde cinema. The film archive and theater is located at 32 Second Avenue on the southeast corner of East 2nd Street, in a New York City historic district in the East Village neighborhood of Manhattan.
The Cinema of Transgression is a term coined by Nick Zedd in 1985 to describe a New York City–based underground film movement, consisting of a loose-knit group of artists using shock value and black humor in their films. Key players in this movement were Zedd, Kembra Pfahler, Tessa Hughes-Freeland, Casandra Stark, Beth B, Tommy Turner, Jon Moritsugu, Manuel DeLanda, David Wojnarowicz, Richard Kern, and Lydia Lunch, who in the late 1970s and mid-1980s began to make very low-budget films using cheap 8 mm cameras.
'The Chicago Underground Film Festival (CUFF) is an annual international festival dedicated to the exhibition of underground and avant-garde cinema, video, and performance. It was founded in 1993 by Jay Bliznick and Bryan Wendorf as an alternative to the mainstream film festival circuit, which was increasingly dominated by distributor product."
Wavelength is a 1967 Canadian-American short subject by experimental filmmaker and artist Michael Snow. Considered a landmark of avant-garde cinema, it was filmed over one week in December 1966 and edited in 1967, and is an example of what film theorist P. Adams Sitney describes as "structural film", calling Snow "the dean of structural filmmakers."
Ron Rice was an American experimental filmmaker, whose free-form style influenced experimental filmmakers in New York and California during the early 1960s.
Marie Menken was an American experimental filmmaker, painter, and socialite. She was noted for her unique filming style that incorporated collage. She was one of the first New York filmmakers to use a hand-held camera and trained Andy Warhol on its use. Her film Glimpse of the Garden was selected for preservation in the National Film Registry by the Library of Congress.
Storm de Hirsch (1912–2000) was an American poet and filmmaker. She was a key figure in the New York avant-garde film scene of the 1960s, and one of the founding members of the Film-Makers' Cooperative. Although often overlooked by historians, in recent years she has been recognized as a pioneer of underground cinema.
The Millennium Film Workshop is a non-profit media arts center located in New York City. It is dedicated to the exhibition, study, and practice of avant-garde and experimental cinema. It was also where the St. Mark's Poetry Project began. Ken Jacobs stated in 2013 that he chose the name Millennium "...because it would have to be that to actually give out equipment, education, space to work in, etc. for free. Dictionary definition: 'A hoped for period of joy, serenity, prosperity and justice.' "
The American Film Institute Award for Independent Film and Video Artists, subtitled and generally known as the Maya Deren Award, was an award presented to filmmakers and video artists by the American Film Institute to honor independent filmmaking. Named for the avant-garde experimental film artist Maya Deren, it was given from 1986 through 1996.
Jerry Jofen (1925–1993) was an American painter, collagist, and experimental filmmaker.
An Introduction To The American Underground Film is a book by the American author Sheldon Renan. It was published by Dutton in 1967. It was the first book about Underground Film.
Anticipation of the Night is a 1958 American avant-garde film directed by Stan Brakhage. It was a breakthrough in the development of the lyrical style Brakhage used in his later films.