Also, if you own Garry's Mod, visit this page to get the most out of the game (all kinds of downloads, I recommend Wire Mod, PHX v2, Smart Snap and Tiled Mod. (Extract them to Steam/steamapps/user_name/garrysmod/garrysmod/addons)
I play GMod. I own CS:S and the orange box but I don't like any of those. I also have Synergy, Empiresmod, and Age of Chivalry although I don't play those very much.
I use my cousins account because when I bought gmod he already had counterstrike. Since then I put orange box on it and bought him a new copy of CS:S, but Steam's crappy support won't let me change it.
Heres a list of the SVN's you should download. Note: Don't just download them all. Many addons are unstable, and some (like Damage Combat System) will be very annoying (they let props be destroyed by missiles)
ULX & Ulib (if you run a server)
Spacebuild must have's:
Res. Distrobution 2
Life Support 2
Life Support 2 Gas Mod
Life Support 2 Res. transit
Spacebuild Model Pack