GameTap
 

Play Free Games Now or Download Games to Own

You need to sign up and download a toolbar for your browser (IE and FF only) to join the fun.
Some games are playable in the browser itself (Earthworm Jim) others you're obviously going have to download and install on your PC (Stalker)

Maybe you should mention that you need to sign up for a 'free' trial.

Yup, thanks for the mention TCD, added - also you need to download a toolbar for your browser (IE and FF only)

Gametap was brilliant until metalobi took over it i had dozens of great free games on my laptop from there but then they got took over and anything made within the last 10 years was taken off the free list :S