Call of Duty - Call of Duty is a first-person and third-person shooter series by Infinity Ward.
Vainglory - Multiplayer Online Battle Arena (MOBA) made for mobile
Quake - Quake is a series of first-person shooter video games released by id Software.
Pairs.one - No-registration multiplayer online memory game
Xonotic - Xonotic is a free (GPL), fast-paced first-person shooter that works on Windows, OS X and Linux.
HaloMD - Play Halo 1 multiplayer online for free (on your Mac)