Counterparty protocol is fully working from months, see
www.counterparty.co.
Also mastercoin and nxt have asset exchanges built in... but atm you can only buy/sell assets against their native currencies (could be wrong, has been a while until i checked)
And then there is ripple.com... it‘s the faster and older one, but whether or not you can call it p2p is a fascinating matter.
THousands of other projects on their way