1. Dungeon Crawl Stone Soup - My favorite roguelike, you would be stupid to not at least try it. It can be downloaded or played in a browser or via ssh/telnet.
2. Dwarf Fortress -> Minecraft for adults.
3. Path Of Exile -> The true sequel to Diablo 2.
4. Liberal Crime Squad -> As awesome as it sounds
5. Mari0 - Mario meets portal.
Thank me later.