I have had it since the summer sale but I have turned a few other people onto Garry's Mod.
I love partaking in some Sled Building/Racing and Trouble in Terrorist Town as well as the 100 or so other games/mods that it supports. Sled Build TTT
Worm's Revolution would my 2nd fav for under $10
This post was edited on 1/4 at 11:00 am