OpenTTD is a clone of the Microprose game "Transport Tycoon Deluxe", a popular game originally written by Chris Sawyer. It attempts to mimic the original game as closely as possible while extending it with new features.
Chicken Invaders 4 Easter pits the player against invading intergalactic chickens, out to punish humanity for our oppression of their earthly brethren.
MegaGlest is a real-time strategy game for up to eight players with a pseudo historic/fantasy theme and 3D graphics. Modding is easy thanks to simple XML formats and Blender export to the open G3D model format.