If you don't already have HL: Source, you're doing something wrong.
Forget that. Just get the Original or Black Mesa.
As for must have games. Company of Heroes and Spec Ops: The Line and both on sale at the minute and are both great games. Spec Ops is £5 and CoH is £2.37, though if you're willing to go over £10, the complete collection is £6.79.
Not changing sig till we get a sequel to Freedom Fighters. Started 13/4/2010