Allows you to search Blockchain.info for the selected text. 'nuff said!
Installing the source is as easy as going to [wrench icon] -> Tools -> Extensions, enabling "Developer mode", clicking "Load unpacked extension...", and pointing to the folder with the source files you downloaded above. Don't trust me if you don't have to!
If you'd like to test it:
17XvU95PkpDqXAr8ieNpYzSdRDRJL55UQ8 (Gavin's donation address)
7a22917744aa9ed740faf3068a2f895424ed816ed1a04012b47df7a493f056e8 (Bitcoinica heist transaction)