EDIT: Apparently only the placeholder sprites (not the fancy ones in the screenshot) have been released under CC-BY-SA. The game should still be 100% playable with these, though.
Very nice new movie from the Xonotic community:
It gives a pretty good idea about the vanilla game mode, although the very popular (some say too popular) "Minstagib" (one shot kills) and the total conversion mod Overkill (All new player-models and weapons, with team-spawn etc.) are also shortly shown.
Not shown are the popular vehicle mod and the recently added lightning gun (still WIP), but you can try everything yourself easily by getting the auto-downloader of the latest development build here.
As with most FOSS games, they are looking for more developers (& players) and if you have some cash to spare you can currently also pledge for the creation of some NPC characters for a single-player or COOP-mode here.
Speaking of FOSS crowd-funding, another great FOSS arena shooter, Red Eclipse this currently preparing a campaign for porting their somewhat aging Cube2 engine to the much nicer Tesseract (which is technically something like Cube3).
Oh and a new version (1.5) but still with the old engine should be out very soon ;)