Team Fortress 2

This is a fairly old game like the previously mentioned counterstrike, but can still be a lot of fun. There are many different game modes and classes you can choose to play as. The most popular game mode is similar to a capture the flag style. You must infiltrate the enemy base, retrieve their intel, and bring it back to your base. There are many different classes to play as, pyro (fire based), engineer (builder), medic (healer), heavy (machine gun/tank), etc. Within the last few years, this game became free to play on steam so the number of players has increased dramatically as as well. For some reason, what this game is most known for is the accessories you can find. There are many different hats and gear you can acquire just by playing that give different buffs to the characters. This helped create many other mods for other games to add in hats and skins.