For some time I've been making a list of freeware titles that are closely modelled after well-known, classic games, or outright started as freeware clones of certain games. Here's what I've got so far:
Arcade games:
Rocks'n'Diamonds - a game that combines elements from
Boulder Dash,
Emerald Mine,
Supaplex and
Sokoban.
Artillery games:
Atomic Tanks - a
Scorched Earth clone.
Hedgewars - a
Worms clone about hedgehogs.
WarMUX - another
Worms clone.
First-person shooters:
Alien Arena - a multiplayer game that follows the best traditions of
Doom,
Quake and
Unreal Tournament.
AssaultCube - a
Counter-Strike-like multiplayer game that uses the
Cube engine.
Blasphemer - a project aimed at creating a free data set for a
Heretic-like game to be used with
Doom engine source ports, much like
Freedoom.
Cube - a game like
Doom or
Quake, but more oriented towards open areas. Has lots of single- and multiplayer games.
Cube 2: Sauerbraten - a sequel to
Cube with more features and various improvements.
Freedoom - a free data set to be used with
Doom source ports to create a free equivalent of
Doom.
HacX - originally it was a commercial add-on for
Doom II, later released for free, and recently it had become stand-alone, no longer requiring
Doom II to be installed to play.
Harmony - an interesting FPS that uses the
Doom engine and has some artistically done graphics and music.
OpenArena - a free data set to be used with the
Quake III Arena engine.
Managerial strategy games:
OpenTTD - a
Transport Tycoon Deluxe engine recreation that allows to use custom data sets, effectively making it a free game.
Miscellaneous games:
Crimelife,
Crimelife 2,
Crimelife III -
Grand Theft Auto series clones.
Frets on Fire - a
Guitar Hero clone.
Platform games:
1213 - a platform game by Ben "Yahtzee" Croshaw made in the style of
Flashback.
Hurrican - a tribute to the
Turrican game series that uses nice pre-rendered graphics.
Lost Snowmen - a hybrid of
The Lost Vikings and
Ren & Stimpy.
Return of Egypt - this game is very much like
Castlevania series games, taking place in Ancient Egypt.
Puzzle games:
Pingus - a
Lemmings clone with penguins.
Real-time strategy games:
0 A.D. - an
Age of Empires lookalike, still in development. Very promising. There's a playable alpha available.
Arrakis - a real oldie, inspired by
Dune II and
Command & Conquer by Stefan Hendriks, author of
Dune 2 - The Maker.
Bos Wars - a sc-fi strategy that uses the
Stratagus engine and looks somewhat like
KKnD.
Commander Stalin - something along the lines of
Command & Conquer: Red Alert.
Dark Hive - this game resembles
Dark Colony somewhat.
Dark Konflict - a
Command & Conquer clone made in 3D with features like day/night cycle and some other enhancements.
Dark Oberon - a game that strongly resembles
Warcraft II, with all sprites having been digitized from clay models.
Glest - a fantasy/steampunk 3D strategy game in the style of
Warcraft III. There are many modifications and enhancements to the original game available.
Priority: Survive - a real-time strategy that looks
a lot like
Starcraft and generally has excellent graphics.
Unknown Horizons - a "colonization" type real-time strategy that follows in the footsteps of the
Anno game series.
Widelands - a
Settlers II clone/freeware counterpart.
Simulation games:
Dolphinity Racer - a racing car simulator not unlike
The Need for Speed.
FlightGear - a freeware equivalent of
Microsoft Flight Simulator.
Oolite - an
Elite clone.
Turn-based strategy games:
The Battle for Wesnoth - this well-known freeware title combines elements from console games
Warsong and
Master of Monsters.
C-evo - a
Civilization-like game.
Freeciv - a freeware counterpart to
Civilization II.
Freeciv.net - the online version of
Freeciv.
FreeCol - a
Colonization clone.
FreeLords - a
Warlords II clone/remake made in Java.
FreeOrion - a 4X strategy game modelled after
Master of Orion.
Legendary Legions - the authors characterize this game as a cross between
Celtic Legends,
Heroes of Might & Magic and
Civilization.
LordsAWar! - another
Warlords II clone.
UFO: Alien Invasion - a game that pays homage to the
X-COM series, but in 3D.
Warbarons - a very nice browser-based
Warlords II clone.
The Wargame Project - a medieval fantasy strategy that resembles
Warlords but has a lot of innovative features making it much more complex.