Liero is high up on my list. The original wasn't open source, of course, but a really good game with a few bugs. The author lost the source code in a crash and that precluded any future bugfixes. Eventually someone reimplemented it faithfully! I love FOSS! ---- OpenMW is my current vice, but obviously content is not open source.
I remember having many good days with classmates in our computer high school class "studying" thanks to LieroX