Giants Citizen Kabuto - years ago it was a big hit; scenery was amazing - tropical islands in variety of sizes with really stunning sound effects - that could keep you wandering around (also, maps were open, dome shaped with isle in the middle, if I recall correctly), as plot rolled you could play 3 different characters/species/classes, Planet Moon custom engine supported Glide, OpenGL and Direct3D and game itself was buggy, had no saving feature (only checkpoints) but you could live with it once you dive in. And there was of course good humor and Delphi who was running topless in full version (at least in European variant). I had not much of chance to test multiplayer back then but you could run it just for giggles alone and test, play stuff before you reached certain moment in story mode; I'm pretty sure that most devoted fans are still making custom maps, mods and keep running servers. If someone had fun with it back then and still got the CDs, you can check community site.

Planet Moon released two demos of one full mission featuring Meccaryns and Delphi; game still should be able to run on modern systems without problems (tho OSX version won't run because it was created for PPC machines) and it's available on gog for almost 9€/10USD