Off the top of my head:
Quake1/2/3, Shogo, Payback, Trapped2, NemacIV, Earth2140, Napalm, Nightlong, ScummVM and all the games that covers, Duke3d, Genetic Species, AlienBreed3d2:TKG (with RTG patch), Gloom3, VirtualGP, Final Whistle, Descent FreeSpace, Exodus:The Last War, OnEscapee, Flashback (via Reminiscine), Another World (via RAW).
There's plenty of others, but for the most part I only mentioned commercial titles, or engines that can play commercial titles. (although of course some of these commercial titles are open source too). Personally I find the vast majority of original open source games to be rubbish, but there is a small percentage of gems amongst the junk. Oh, additionally Ive listed some 68k games that Id imagine should work, but may not. As for Gloom, earlier versions may work, so it may be worth looking into, but I know for sure Gloom3:Zombie Massacre is definately RTG friendly.
