Try Super Meat Boy. By the makers of Binding Of Issac. Amazing Platformer.

I have played that and 100% on 360. A very amazing game :D!

Defender's Quest, Dungeons of Dredmor, and Cthulhu Saves the World.

Problem with Canirunit.com is that for whatever reason it thinks my PC is basically a potato. It says I don't even reach the minimum for Issac or Terraria, yet I can run them flawlessly. And I will check those games out, thank tmw :D

Good ol' Starcraft & BroodWars... Unless the code has been significantly updated (not sure if you could run the new ones) it onl required Win98 or NT4.

DefenseGrid: The Awakening is a great low-resource tower defense game.

The original Half-Life should work perfectly for you. TF2 worked fine on my Pentium3 650Mhz although its grown a bit over the last few years.

PeggleExtreme (Peggle with Valve-art and themes) is worth playing too.

Just depends on what you're into.

you should try try awesomenauts its a pretty good game

Defender's Quest should run and it's actually an amazingly good game that will give you dozens of hours of entertainment. The devs are also responsive and communicate a lot.

Most 2D games should work fine. Hard to be sure without knowing any specs, though.

You may be able to play run Castle Crashers. Fantastic game (with a controller (keyboard controls are hard to get used to))!

Killing Floor is a 3D game but it can run on a calculator.

I have played Call of Duty 4: Modern Warfare on an old laptop with integrated graphics once.

magic the gathering 2012, containment a zombie puzzler. I play both of these on my crappy laptop.
EDIT: oh and snapshot.
EDIT: Magic is on daily sale 2012 and 2013.
http://store.steampowered.com/sale/wintersale2012_magicthegatheringfranchise

I played 'World of Goo' and 'Plants vs Zombies' on my crappy Netbook... Both are awesome.

Any good games for a PC with low HDD space, apart from basic platformers etc?

go back in time. games like deus ex, ut99 and 2004, quake 3 can run on a netbook

I feel your pain. It is the frustrating truth that game are obsoleted so quickly. You can play a 30 year old Monopoly game from a box, and you will in 30 more years, but computer games are discarded quickly by developers. I started with 'Quake2' in 1998, and began aquiring all of the Quake games. At the same time, I hold off on mew Microsoft platforms until the majority of bugs are addressed, so by the time I got Win98 SE, Quake was already incompatible, and my discs were worthless. You need to exceed the minimum requirements at least, and backward compatibility is an unaddressed concern by game manufacturers and OS developers. Go online and play flash games. Computer game makers lack integrity, or customer commitment. I am finally able to replay the Quakes, because Steam made them playable again. Stay away from Ghost Recon Adbanced Warfighter 1 though...