Anything, to be honest- just great games I've missed out on.
Well then...
System Shock 2 and Deus Ex are two of the best turn-of-the-millennium PC games out there, and will run on just about anything these days. Both are very deep games that make up for in worldbuilding and gameplay what they lack in graphical quality. Check out Thief 2 while you're at it, it's a great stealth game from the same era. And Half-Life too, if FPSs are your thing.
Moving forward a bit, the original Far Cry is punishing but tremendous fun. Vampire: Bloodlines has been praised to high heaven and although I never got into it I've had people whose opinions I respect recommend it to me over and over. F.E.A.R. is a great FPS from the mid 2000s that genuinely scares in places.
Games like Hard Reset and Painkiller are pure run-and-gun adrenaline action for when you need it. Star Wars: Republic Commando is one of the few Star Wars games worth playing in my opinion, although I've heard that the Jedi Knight games do a very good job of making you feel like a Jedi (lethal, but one slip-up and you're gone).
FTL: Faster than Light is an easy-to-grasp, difficult-to-master spaceship simulator that I've lost too much time to for what it is.
That's all I can think of off the top of my head. Quite possibly you've already played a bunch of these! All of them are old or low-spec so should run acceptably on even a lower-end laptop these days (with the possible exception of Far Cry, which was 2004's Crysis, and Morrowind, which is hilariously poorly optimised). Assuming, of course, you're playing on PC - I'm a dirt-poor student who hasn't the money for consoles, so I sometimes forget they exist
Hope this helps!
