Sauerbraten (video game)
Sauerbraten was made out of the game Cube, but is much more advanced and a player can create new levels. Sauerbraten was not made to look very good, but to allow easy map making within the game without making a big download. Sauerbraten CTF Edition is less than 350 megabytes.  The game Crysis is around 3,000 megabytes.