Fan game

An example is Tetris, fan games are computer tetromino games unconnected with Elorg, The Tetris Company, or any other licensor of rights in the Tetris trademark. They tend to follow the well-known rules of Tetris but may not strictly follow any official guidelines.

Straight tetromino games
The following games more or less faithfully replicate the rules of some version of Tetris, either pre-Guideline or post-Guideline:
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 * 4-Tris - Recent game cartridge by Joe Zbiciak for the old Mattel Intellivision game console
 * Aint - free software for MS-DOS, possibly the first copylefted tetromino game
 * Bastet - Literally, Bastard Tetris. Made to be as annoying and frustrating as possible.
 * BCTMP - Brian Caos Tempting Music Player (versions Beta 1 and Beta 2 previously known as "Brian Caos Tetris Music Player"), IT/MOD/S3M/XM music player with a built-in Tetris game
 * Bedter by Andrew Bednarz - freeware for Windows and Linux, famously targeted by TTC which alleged "look and feel" copyright violation
 * Blocks A Java version of the game.
 * Block Star- Integrated Facebook Tetris, featuring comparison leaderboard to your circle of friends, or all users.
 * Breakris by Jürgen Müller - freeware for Windows. It has a very good single and multiplayer-Tetris, plus Breakris - Breakout and Tetris at the same time.
 * Cultris
 * Falling Up- a "twist" on Tetris (freeware/open source for Windows, OS X, linux)
 * GinT - Falling Tetromino game within the 3D world of Second Life.
 * Hatetris - Online game that always drops the worst possible piece at any given moment.
 * Heboris
 * Java Tetris v2.11 - Nice Java applet Tetris clone by Andreas Ludwig that supports multi-player and computer AI players
 * Lockjaw - primarily a guideline simulator, with dozens of options to change behavior
 * Lockjaw: The Overdose (formerly Tetanus On Drugs) - free software for Game Boy Advance, inspired by Tetripz and considered by many players to be superior in look and feel to the GBA version of Tetris Worlds
 * Official Scratch Tetris - an official Tetris game on Scratch. It is a lot like TGM, and has Tohu - Roots (it sounds beautiful!), and it is possible to narrow the play field.
 * Quinn - Described as an arcade-style tetromino puzzle game written and designed specifically for Mac OS X
 * TetFun 2000 - Seen on http://liacs.leidenuniv.nl/~kosterswa/tetris/teet.html. See also Animate TetBlox
 * Tetriculis - Tetris as Google Gadget.
 * Tetripz by Mute Fantasies - freeware for MS-DOS, with visual distortions of the playfield
 * Tetris - the son of Tetris Best
 * Tetris Best - Tetris's dad. Its rotation system uses 24 wallkicks.
 * Tetris Best 2 - a remix made by MathBoy112358, with destructive hold and climbing pieces (glitch?)
 * Tetris Brick - score increases by itself
 * Tetris Perfect - successor to Tetris Best
 * Tetris Return - Fixed a slipping flaw from TPRS by rearranging the 3-kicks.
 * Tetris Weightlifting - A modified version of Tetris using exercise equipment as the control mechanism.
 * Tetrominoes A light and fast Javascript version of the game
 * Texmaster - possibly the most accurate PC-based simulator of the TGM series
 * Vertris- A simple cross-platform clone.
 * Zentris - by Robert J. Sundling

Variants
These games have game play rules inspired by Tetris, such as rotation of shapes to form complete lines, but introduce rule variations that change the game play greatly.
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 * 3DTRIS - 3dtris.de A clone of Blockout (3D variant of Tetris) in Macromedia Flash. Its gameplay is very smooth, just like original Blockout. Online high score list with growing competitions.
 * Blockout II - blockout.net The best clone of Blockout (3D Tetris) for Windows using DirectX. Online high score list, forum, wiki.
 * Quadra - Open source clone with cascade gravity. Features very fast-paced online play.
 * Java Tetris - A Java Tetris game that follows the gameplay rules of the original, but has more than 30 shape types and a few new game modes, including a "Hardcore" mode where the game is played on a 125x125 grid.
 * 3D Tetris - 3dtetris.com 3D variant of the classic Tetris game. You can play the game: online, offline and mobile. The game use the Java virtual machine.
 * Tetrinsanity - Also made by MathBoy112358, no T piece, and has a big gap in left side.
 * TETRIS MAX - a stacking game, with gaps and special rotation system
 * TETRIS MAX 2 - made by same guy, with more gaps and special I rotation
 * Tetris Worst - The other successor to TBII, with 20G gravity and bad rotation/movement
 * Tetris Worst 2 - Made by Piotr Grochowski. It uses creepy music and upward gravity
 * Tetris Worst 3 - Here's a MathBoy112358 Tetris fan game again! This time the pieces disappear, just like in TGM.

Comparison of notable fan games

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