Half-Life 2 [Orange Box], PC - £23

Half-Life 2 [Orange Box], PC - £23 delivered

This has got to be one of the best value deals in gaming ever. Seriously.

Halflife 2
Halflife 2: Episode 1
Halflife 2: Episode 2
Portal
Team Fortress 2

That's under a fiver each.

Even if you already own HL2/Episode 1, they are different serial numbers, so you could theoretically sell/give away HL2 and Episode 1 to someone. I am pretty sure this is cheaper than it would be on steam (maybe even cheaper than buying just Episode 2/Team Fortress 2/Portal)