Garry's mod is a really fun game, there are lots of game modes you can play with friends! You can do lots of things, your imagination is your limit...

Unturned is a apocalypse game, best played with friends. You must fight through the zombie apoc meeting new players and finding new weapons and cars!!!

Rust is a good game, a nice survival game playable with mods, it is good to play with friends and they post weekly updates and add awesome things!