Risen is great fun, but only if you like earning what you are after. and I mean seriously earning it, I've spent 8 hours (leisurely) trying to become a Mage and have yet to cast a single spell.

Fallout 3 - In my opinion Fallout 3 was awful (especially if you liked the first two), just a crap mod for Oblivion, Combat is far too easy - there is far too much ammo in the world and VATS just makes a bad system worse, the world is bland and repetitive giving little impetus to explore, and the main quest is short and uninteresting. Oblivion is much better, if still flawed

Oblivion - I have sunk many, many hours into Oblivion (with mods), just ignore the main quest once you have got about half way through, and just wander, and find all the hidden shrines and towers and villages and other random things of interest. Combat is....dodgy, people seem to dislike the melee combat but both Magic and Archery make the game very easy, but the world is very engrossing and more than makes up for it. About the Mods, I recommend playing through the game normally for a bit first, the Mods are good, but many ramp up the difficulty so until you are used to the game and combat don't worry about them.

Burnout Paradise - This game is a blast to play, but if you looking for anything other having fun speeding and crashing cars then you won't find it here. So yes good for gaming where you don't want to have to think too much.

The Orange Box
Risen
The Elder Scrolls IV Oblivion (I've heard a lot of reviewer praise, and a lot of player criticism)
Fallout 3
EVE Online
Company of Heroes

Edit:
Burnout Paradise
Sims 3

never tried

never tried

Fun on xbox360 with DLC however dungeons become repeatitive and eventually it becomes boring

never tried

@ EVE DO NOT PLAY IF U HAVE A SOCIAL LIFE, you also need to get the box with the special plane in the box its a shuttle worth 100million credits or whatever they use I forget some jerks will try to buy it from you for 50k there ripping you off, on eve there are a lot of camper pirates and they get annoying if ur trying to move things when ur alliance is offline.

COH is like dawn of warjust replace with ww2 units

there are mods for oblivion which changes the leveling system to an exp based one (like in dnd,  wow or diablo ect.):

http://planetelderscrolls.gamespy.com/View.php?id=4425&view=OblivionMods.Detail

-i would not buy empire instead wait for napoleon total war (or buy mediveal2 total war for extremly good mods, like the third age total war mod)

-you should try the demo of demigod first, and only buy it if you are into multiplayer games (the single player is next to nonexistant)

You've just saved my ~$20 investment, thanks

Dunno about Demigod as I've only played the demo, but Empire is awesome IMHO. The AI is beyond moronic however, both strategically and tactically, but the game itself is very deep and there's a great joy to be had in trying real-life tactics in it even if the braindead AI makes them all innecessary.

But I also second the recommendation for Medieval II and add Rome to the mix, M2 is, despite its age, one of the best looking games ever IMHO and the gameplay is classic Total War fun in a Crusades-themed package, and while Rome is a bit more dated it's also the only one with a decent strategic AI (tactically its the same stupidity as always though), and the roman period holds many interesting units and tactics to try out that had become hopelessly outdated by the period covered by the other two games.

I've had a lot of fun with demigod, recommend you try the demo first and only bother if you are in to multiplayer.

As for oblivion i played about 1/2 - 3/4 of the way through at which point I got bored having realised I had tasted all the possible spells and any knew ones were simply more powerful versions of the exact same thing. Additionally I thought the creeps that levelled as you did was a dumb mechanic, taking the fun out of working on your character.

If your computer crashes or something like that, can you redownload the game?

I bought Risen, and am having fun exploring their world.  It's a single player RPG without much combat dynamics, but I'm only level 2 in the game.

I'd also recommend watching Dragon's Age: Origins.  BioWare will be releasing that November 6.