http://blockexplorer.com/Armory
And there are a few others, I believe.
I also have a little AHK script (which auto-refreshes the address balance from blockchain.info)
that I made for my own use (Windows only.) I will consider cleaning it up and releasing it.
Does Armory allow tracking of Paper Wallets already printed?
Yes. It requires tons of RAM and local storage of the blockchain, though.
Another option that comes to mind is Electrum - with a little tweak, you can track a public address without importing its private key.
Mind explaining to me how I can tweak this up?
Sure:
- First, import a dummy address. For instance one that you have generated at bitaddress.org.
- Close Electrum and open the wallet file in notepad.
- Look for the dummy address and replace it with the address you want to watch.
- Save and exit, and launch Electrum again.
- Presto!
Notes:
1. You can leave the private key of the dummy address unchanged - it won't do any harm.
2. Yeah, it's just a quick and dirty trick, but it works. Hopefully Electrum will implement true
watch-only wallets some day.