Balder is a multiplayer Zero Gravity First Person Shooter using the Crystal Space engine. It also uses the borax template and the GNE -- Game Networking Engine. It will emphesise team cooperation, instead of the standard frag-fest and meaningless violence of most first person shooters. As one will not be able to change directions mid flight, one must carefully consider how to get from point A to point B without getting "flashed" (players carry flash pistols which freeze opponants). For a more I suggest reading the Design Docs, especially the "General description" section.
The inspiration behind Balder is from Orson Scott Card's novel "Ender's Game".