Check Bitcoin address balance

I know blockchain.info has an API:
gets the bitcoin address’s balance, but it’s not open source and must connect to the Internet.

And I find the open source website blockexplorer.com has not show the right balance:
It gets 0 but not 1250000 from blockchain.info.

I tried bitcoin-qt client debug console:
getbalance 134dV6U7gQ6wCFbfHUz2CMh6Dth72oGpgH
But it returns 0 whenever the address is not mine.

How can I get someone else’s bitcoin address’s balance?
The bitcoin database is big and hard to check every transaction to get the balance.