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.

Bitcoin address balance

Bitcoin address info

Bitcoin address data

Bitcoin get address balance

Bitcoin Address Utility