Games on this list were included based on recommendations, if you'd like a game added to this list, please post it on the sticky of /f/Games. For a more comprehensive list, please see this Wikipedia page.

Adventure/RPG

Arcade/Casual

Art

Hummingbird - Infinite procedural musical exploration game.

Passage - A simple experimental side scroller game intended as an abstract metaphor for the human condition.

Artillery

Apple Flinger - Angry Birds-clone for Android.

Atomic Tanks - Multi-platform Scorched Earth clone similar to the Worms series.

Cat-O-Pult - Castles/Worms type of a game, but in this one you use cats as your weapon of choice.

Gusanos - Clone of the DOS game Liero.

Hedgewars, Warmux and OpenLierox - Clones of Worms.



Board/Card

City Builder/Tycoon

CorsixTH - Clone of Theme Hospital

Freeserf - Clone of The Settlers 1/Serf City.

FreeTrain - Rail and business simulation inspired by the A-Train series of games.

KeeperRL - Dungeon management and roguelike.

LinCity-NG, Micropolis Web and 3d.city - Clones of SimCity.

OpenSkyscraper - Clone of the simulation game SimTower.

OpenTTD (GNU/Linux, Windows & Mac) - a transport simulation game based upon the popular game Transport Tycoon Deluxe, written by Chris Sawyer.

OpenRCT2 (GNU/Linux, Windows & Mac) - An open source reverse engineering of Rollercoaster Tycoon 2. Requires assets from the original game, acquirable here

Fight

OpenOMF - Remake of One Must Fall 2097.

Flight

FlightGear - Flight simulator.

Triplane Classic - Sidescrolling dogfighting flying game

FPS/TPS

Infinite runner

Biketris - Hybrid of block-stacking puzzle and endless runner.

Red Runner

Metroidvania games

Birdsong - Game about a bird that is about to leave the nest.

Meteorite - Lowres FPS, inspired by Metroid Prime.

The Diasporic Crypt - Libre game inspired by Castlevania, where you play a vampire who moves around in a portable teleporting catacombs underground.

MMORPG

Music

Physics based

Puzzle

Racing/Driving

Roguelikes / Roguelites

RPG

Side-Scroller/Platformer

Programming

Robocode - Programming game, where the goal is to develop a robot battle tank to battle against other tanks in Java or .NET.

Puzzle

Puzzle-platformer games

GlPortal - Clone of Portal.

Mari0 - Odd mix of Mario and Portal.

Minilens

Nova the Squirrel - NES game with ability copying and puzzles.

Sandbox/Voxel

ClassicalSharp

Craft

Freeminer

Gnomescroll, ClassiCube and Freeminer - Minecraft-clones.

Lamecraft

Manic Digger

Minetest - (GNU/Linux, FreeBSD, Windows, Mac & Android) - Uses C++, Requires mods to be fun

OpenSpades - Clone of the team-based first-person shooter and voxel-based sandbox Ace of Spades.

Terasology - Fancy lookin' Minecraft

TrueCraft - Clean-room implementation of Minecraft 1.7.3

Voxelands - Minecraft standard clone?

voxel.js - Tool for creating minecraft clones

Sealth games

Art Treachery - Stealthy art heist game.

The Dark Mod - Fan-made spin-off the the fantastic Thief series (Not suitable for low-end systems)

Shoot 'em up

Social simulation

Cart Life - Simulation game in which the player controls one of three street vendors, and attempts to run their shop whilst looking after their health, interests, and families. Source code

Coming Out Simulator 2014 - Interactive game intended to help LGBT youth to understand their sexuality.

Our Personal Space - Marriage survival sim set on a space colony in the near future.

Two Interviewees - Short visual novel about the gender gap.

Space Combat/Sim

Sport game

Bygfoot - Football/soccer management game.

Classic 8-Ball Pool - Pool game written in JS and HTML5.

FooBillard - OpenGL-based billiards simulation game.

Goal Rush - Fast paced arcade game where two-robots team will face each other to score the maximum of goals during short matchs.

YSoccer - Retro style, multi-platform soccer game.

Strategy/Tactics

Vehicular combat

Armagetron Advanced - Multiplayer 3D clone of Tron.

BZFlag - 3D online multi-player tank battle game.

OpenC1 - Remake of Carmageddon

Walking simulator

Uncategorized