It's just on Ripple trade (http://www.rippletrade.com) at the moment. In your Ripple account, click the "Exchange" tab and then "Trade". Limited levels of liquidity are available for WLD/ XRP and WLD/ BTC (snapswap.btc). New places where WLD can be traded will be introduced, along with additional platforms where 10,000 WLD can be issued and used for payments.