People basically taking a rules system and stripping out the setting info and gearing it toward a different genre or setting. There is a Star Wars hack for Dragon Age for example. Often they have a small rules set you use in addition to the original or in place of.I've seen the word "hack" used recently with games. What does it mean? It seems to have snuck up on me without explaining itself!
I've heard it mentioned in a couple of places, but here's the first item off the google search: http://www.indie-rpgs.com/forge/index.php?topic=28410.0E6 counts. Star Wars hack of Dogs sounds interested. Nytmare do you know where it might be found?