Samizdat

"Samizdat: I write it myself, edit it myself, censor it myself, publish it myself, distribute it myself, and spend jail time for it myself."

—Vladimir Bukovsky, To Build a Castle: My Life as a Dissenter

Samizdat (Russian, самиздат, lit. "self-publishing") originally was a Soviet-era practice in which material which was censored or otherwise could not be officially printed (for political or other reasons) was reproduced via other means, such as mimeograph, typewriter or even handwriting and distributed sub rosa, so as not to attract the attention of the authorities.

In a broader and more modern sense, Samizdat is any unpublished work which is reproduced and distributed by hand or other low-tech method.

Not only has the rise of the Internet has rendered samizdat somewhat obsolete -- Social Media, Bulletin Board Systems and other electronic communications have made the distribution of "unofficial" material trivial and easy -- but it has also stripped it of much of its countercultural aura of rebellion.

Contrast with Doujinshi, which is also self-published but fills a vastly different hole in the social fabric.

Examples of Samizdat include:

Computing

Hacker culture had its own definition of samizdat, as found in the Jargon File:

The process of disseminating documentation via underground channels. Originally referred to underground duplication and distribution of banned books in the Soviet Union; now refers by obvious extension to any less-than-official promulgation of textual material, esp. rare, obsolete, or never-formally-published computer documentation. Samizdat is obviously much easier when one has access to high-bandwidth networks and high-quality laser printers. Note that samizdat is properly used only with respect to documents which contain needed information ... but which are for some reason otherwise unavailable, but not in the context of documents which are available through normal channels, for which unauthorized duplication would be unethical copyright violation.

After Bell Labs changed its UNIX license in 1979 to make dissemination of the source code illegal, the 1976 Lions book which contained the source code had to be withdrawn, but illegal copies of it circulated for years. The act of copying the Lions book was often referred to as samizdat.

Literature

The first full-length book to be distributed in the Soviet Union as samizdat was Boris Pasternak's 1957 novel Doctor Zhivago.
Everything Flows by Vasily Grossman, written after Soviet authorities seized his masterwork Life and Fate, was distributed as samizdat before the manuscript was smuggled to the US for formal publication in the late 1960s.
The Butterfly Kid originally circulated around New York City in a mimeographed manuscript form for a year or so before it finally saw actual publication in 1967.

Video Games

In Deus Ex: Mankind Divided, you have to do a mission where you have to check out an underground news organization called by this trope.

Real Life

Before the internet, Amateur press associations were the paper equivalent of a modern email mailing list, and allowed small groups of enthusiasts to discuss their common interest in the form of a mailing compiled from contributions sent by their members to the APA's manager/editor, who assembled them into issues which were then sent back out to the APA's members.
Similarly, early science fiction fandom abounded with low-circulation fanzines (from "fan magazine"), sometimes having "print runs" no larger than a dozen or so copies per issue.
"Ribs" (Russian, рёбра) were records pressed on used X-ray film during the era of the Soviet Union, as a means of bypassing censors and their crackdown on Western artists.