Twenty five quid doesn't necessarily buy you a lot of game these days... but thanks to the Steam Summer Sale, it will buy you every game Valve has ever made, plus expansions.
I paid that much for the Orange Box alone. And it was still worth it.
Just in case you don't already feel the urge to cough up the £25, here's a few games to get you going: Portal, Portal 2, Half Life 2, Half Life, Left 4 Dead... and every expansion pack. Considering that Portal 2 is £14 by itself, this has to be worth a punt.