You can use blockchain.info API:
URL:
https://blockchain.info/ticker
Response:
{ "USD" : {"15m" : 478.68, "last" : 478.68, "buy" : 478.55, "sell" : 478.68, "symbol" : "$"} }
Returns a JSON object with the currency codes as keys. "15m" is the 15 minutes delayed market price, "last" is the most recent market price, and "symbol" is the currency symbol.