I love Garry's Mod. It is very addicting, fun to play with friends, and being able to do what you want is spectacular.

User Rating: 10 | Garry's Mod PC
This review is very short. Garry's Mod is not a game. It is a physics toy and sandbox style tool. In Gmod, you can reluctantly do whatever
you want to. You can: build things such as cars, buildings, bunkers,
airships, boats, or whatever you can think up, use weapons from other valve games, as well as add-on weapons created by
other people, play with physics, kill NPCs with literally anything, drive a world of vehicles, paint, destroy stuff, spawn props, spawn ragdolls to pose and beat up, make a machinima for use on Youtube or other video websites, listen to music, and MUCH MUCH more. There are addons to download, like maps and player models.
The maps can vary from Counter Strike to Team Fortress, Portal to
Garry's Mod, and Day Of Defeat to Left 4 Dead. A very addicting toy from Team Garry.