There are multiple online Services which can do this.

For Example Blochain.info:

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:

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.

There are multiple online Services which can do this.

For Example Blochain.info:

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:

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.

Another example is biteasy.com's REST-API:

https://api.biteasy.com/blockchain/v1/addresses/12sENwECeRSmTeDwyLNqwh47JistZqFmW8