Darkstar itself is a a scalable backend running server side Java, and comprises the servers and the service together. The binaries are free to download and play with, and is in the process of being run past the lawyers to get GPLed. Sun wants to have it open top to bottom, but that will take a few more months.
The service has Java, C++ and assembly wrappers, so you can write your client or the server code however you want. It also allows you to put 3rd party plugins on to it, so you are not tied to any specific widget they have.
One of the best features of Darkstar is channels. You can pipe information around in a sane fashion, and the service gives you a single threaded view of a multi threaded problem. It is game agnostic, you can write a video conferencing back end or an FPS, it doesn't care.
Whatever you write, Darkstar also does object persistence for you, again no need to write the messy plumbing, it is there. It also has features for compartmentalized server side cheat detection. If you see someone doing something out of bounds, you can kill that on the server and not expose other clients to the cheating.
Basically, Darkstar is plumbing for servers with a hopefully friendly API. It is not directly tied to any game, but it is tuned for a lot of the things games often need. Sun will give it to you for free, and if you like it, they will sell you as many pre-configured servers as you want with it installed. Plug and play, pun pun.
If you are not into owning walls of servers and running data centers, Sun will do that for you at what it claims are very reasonable prices. Pick your method of delivery, you own it, it owns it, or it has nothing to do with it. In any case, it is GPLed, so we all win. µ