Overlord is fun. A hybrid/mix of RPG, strategy, action, and adventure elements. Some light humor, and a bit of replay value (but not over-much). Minimal choices-and-consequences.
Hellgate: London (with the Revival Patch) is insane fun. A new Revival patch is coming out in the next month or so (at least, I hope), I'm really excited for it. Yes, the game is now single-player only. But that's okay, I like single-player games. There are some other great mods for the game that I'd recommend using. It's a hack-and-slash RPG. Some say the plot is threadbare and barebones, but I was able to enjoy it aside from one or two gaping holes and a few odd leaps of logic. Everyone says the game is rubbish, but I don't think so. Best $2.50 I ever spent. I want to make a Cabalist Evoker for my next character, the gear looks sweet. And, uh, shooting lightning and fire out of my hands is going to be fun. You can also play the classes that use guns, switch the camera to 1st person, and treat the game as an fps. I played the Templar faction with a Blademaster class in 3rd person, so my experience is a 3rd person hack-and-slash with melee weapons.
Trine was fun, though it is a pretty short game and has limited replay value for single-player. Very pretty visuals, I ought to replay it soon.
Psychonauts. I got 90% through it on the PS2. PC version has way faster loading times. Hilarious game, very creative and has pretty tight gameplay.
Gothic 1. Even with patches and mods, it can crash now and then. I enjoyed playing it (er, with god mode). It's an Open World RPG, but the class and skill system is tied more to the faction you choose than to free-form character creation. No level-scaling means it's easy to get your butt kicked early on - but in later levels, you'll be mopping the floor with your enemies. If you like G1, you will probably want to get G2 to play.
Titan Quest is fun, and you can still play multiplayer on the Gamespy servers. It's a hack-n-slash RPG set in ancient times (Greece, Egypt, Babylon, Asia) with a whole smorgasbord of mythological monsters and legendary items. The community patch is a must-have, and there are a few nice mods for it, too (especially for freeing up the camera). It is a very long game, and you can go back to previous areas and fight the respawned enemies and bosses to your heart's content.
Majesty. An rts game even I can play! Um, but sometimes I need cheats. All my heroes are such cowards.
I have several other games that I bought and still need to play, so I can't really comment on my personal experiences with them.