Basically, is THE GOLDMINE of online music creation.
What i mean by this is, it's a 100% free-to-use online jam session page.

there's thousands of musicians world wide, positing tons and tons of copyright-free material on a daily basic.
sound quality ranges from anywhere between professional sound engineer quality, to pretty much recorded on a toaster from 1950.

i highly suggest you to go there, and read what they have to offer. you will not regret it!