Counterparty has a functioning decentralized exchange as well as a number of other features. The project is only four months old, however, the development timeline has outpaced all other similar projects, such as Mastercoin. As of now the protocol is accessed via command line, however, Counterwallet will be released shortly and will allow users with little or no technical prowess to use the fully functioning features of Counterparty. Here is a link to the official announcement thread as well as the Counterparty forums.
Announcement thread: https://bitcointalksearch.org/topic/annxcp-counterparty-pioneering-peer-to-peer-finance-official-thread-395761
Counterparty forums: https://forums.counterparty.co/index.php
Not looking to promote here, but Counterparty does have a functioning (if still alpha) implementation of a decentralized exchange (the same cannot be said of any other 2.0 coin yet, though I know several are working on similar projects). Trading bitcoin for IOUs (ala Ripple) currently has not been implemented, if for the lack of a, well, counterparty for that trade. But do take a look.