Simple open source games that are nice and playable.
Eat them all
Tower Defense / RTS,
play (it's not available online, you need to download
and run it locally), video,
MMO RPG, play,
Fantasy Map Tactics
Simple TBS prototype, play,
HTML5 Tower Game Starter Kit
Tower Defense, play,
play (the site is currently down, you
need to download and start it by yourself, but it is working, I played it),
play (currently there's some minor error on the site that
prevents from playing it, but I played it earlier, it was in working state),
The Mana World
MMO RPG, site, C.
Turn Based RPG, site,
Turn Based RPG, site
2D Minecraft clone, site,
Sources with graphics, Android, Java.
A Glorious Escape for A Lich King
sources, Flash (Haxe).
RTS. Multiple games with open source content available.
Shut them up
Flight Simulator https://github.com/Chmood/shmup