anyway, first up...
Garrys Mod 9
This is the free version of the game, gmod 10 is better though cos you get more features, like orange box content etc, but gmod 9 can be interesting as well. They have their own mini mod's within the game, but can just be fun making random shit, then attaching rockets or something to it so it flies all over the place and kills ppl it hits, or whatever. it's a unique game and worth trying out.
http://halflife2.filefront.com/file/Hal ... _Mod;49585
Synergy 1.1
Originally called Tim Coop, this is basically Sven Coop on HL2. You can play through HL2 to the end of HL2 ep2, as well as other coop maps, can be fun for a laugh, and well, can just be fun playing through HL2 with a group and acting like n00bs

http://synergymod.net/
(download link) http://pandatales.org/synergy/synR11Full.exe