Teeworlds

Teeworlds is a free, open-source 2D multiplayer online shooter. You play as a small round character (a "tee") and compete in fast-paced arena matches. Here's a quick overview:

Gameplay

• Side-scrolling 2D arenas with physics-based movement
• A grappling hook for swinging and wall-climbing — mastering it is key to getting good
• Classic game modes: Deathmatch, Team Deathmatch, Capture the Flag

Weapons Pistol, shotgun, grenade launcher, rifle (laser), and a hammer for close-range melee. Weapon pickups are scattered around maps.

DDNet (DDraceNetwork) is the most popular mod/fork, focused on cooperative race maps rather than PvP. It has a massive map library and its own ranking system — many people know Teeworlds through DDNet specifically.