Soldat

Soldat is a unique 2D (side-view) multiplayer action game. It has been influenced by the best of games such as Liero, Worms, Quake, and Counter-Strike and provides a fast-paced gaming experience with tons of blood and flesh. Soldiers fight against each other in 2D battle arenas using a deadly arsenal of military weapons, across 7 default game modes. It features 18 different weapons and 60 maps to frag away on, with full support for user created content.

Reviews:

“One of the most addictive and fast-paced online multiplayer experiences ever created”
Gamespot

“I loved SOLDAT - played the hell out of it back in the day.”
John Romero (id Software)