Freespace 2 was a fantastic but ridiculously undermarketed space shooter, released in 1999. The source code was released a while ago, and there's now a
Mac OS X Freespace 2 port. (You'll need a copy of the original PC version for the data files).
Even cooler, if you don't have a copy of the game the end user licence agreement permits the distribution copies, as long as you don't make any money from it.
