At it's simplest it's a massive free smaple library. But, there is also a sequencer and a MIDI tool (Whatever that is). I haven't tried these yet but they look like they could be of use to those without software.
This is unfortunately the end of my very quick heads up to you guys as I'm off to work now. I'll have a proper play around with it when I get back. In the meantime all of you get an account an see what this place can do. I'm interested to see what people can come up with.
I've had a quick look through the sample database. There isn't really that much there but then again the site has only been up for about a month. I might start contributing some samples if I get the time.