There are multiple online Services which can do this.

For Example [Blochain.info][1]:

http://blockchain.info/address/12sENwECeRSmTeDwyLNqwh47JistZqFmW8?format=json

There you get a field called final_balance and all transactions for this bitcoin address.

If you dont need so many information you can use this [Query API][2]:

http://blockchain.info/q/addressbalance/12sENwECeRSmTeDwyLNqwh47JistZqFmW8

Here you will get only a number as return value which represents the amout of bitcoin in satoshi for this address.

You can also use other sides like http://blockexplorer.com/q which offers equal services.

  [1]: http://blockchain.info/api/blockchain_api
  [2]: http://blockchain.info/q