This is the Quake 3: Arena game engine ported to run on the Xbox.
So what does it do?
As it stands now it will let you play the single player game portion of Quake 3: Arena. You should also be able to play on a network and over the internet, against other Xbox players as well as people running Quake 3 for the pc. For internet play you will require a valid CD key from the pc version of Quake 3.
It also has the ability to load and run some of the many mods available for Quake 3.
But please be aware, there are some mods that will run fine whilst others won't run at all. Unfortunately this is due to limitations in the Xbox hardware (basically it's a very tight squeeze trying to get Quake 3 to run at all using 64MB). I have not tested many mods, really I've done nothing more than try a couple to make sure that the mod loading process is working properly.
If you intend to try running any mods I strongly suggest a usb keyboard and/or mouse be attached to the xbox. Without them it may prove very difficult, even impossible, to navigate through the mod's interface and get a game running. This is because none of the mods available will be setup to work properly with the xbox controller.
If any mod authors do wish to make their mods compatible with this port then please drop me a line and I can let you know what (simple) changes are needed.