Valve Complete Pack, PC - £78.94

Valve Complete Pack, PC - £78.94 on Steam

A big spend, but it is a huge catalogue of quality games. You get:

The entire Half-Life series, including expansions, episodes and multiplayer,
Team Fortress 1 and 2,
Counter Stike, Condition Zero and Source,
Day of Defeat, Source,
Peggle Extreme,
and Left 4 Dead.

Worth it is you don't already own any of them.