Soldat is a 2D action shooter which features an online play mode, aswell as a customizable single-player mode. Single player features modes such as Rambo, CTF, Team Deathmatch, and a free-for-all deathmatch. I would say to make a server, but I haven't played in a long time. I think my name was 'IpecacSportsDrink' when I used to play.
Along with it's client, it features a map-creating program aswell as other developers' softwares.