Endgame: Singularity is a fun (and sometimes very frustrating) small strategy game worth checking out.
The Dark Mod is basically Thief but really well done with a solid amount of (fan) missions to choose from. Speaking of I should play some of it again.
If you haven’t played them the original Fallout 1 and 2 (the Fallout 2 Restoration Project is a big recommend) and Arcanum are classics as well.