So far I've downloaded Garry's Mod, and also this nifty program called GCFScape. GCFScape just allows you to play any sound bits from the valve games such as L4D or TF2, and is very simple to use. Be warned though, there are a LOT of sound bits per game! Linky here.
EDIT: I forgot, L4D sound bits do not require GCFScape to use, they can be played as is.
Edited by Ghostrider, 16 November 2009 - 16:55.