With this tool you can easily verify any bitcoin message. You just have to provide the bitcoin address, generated signature and the message that has to be verified. These informations can easily be generated with your bitcoin-qt client.
If you want a verification for an address you spend bitcoins with and this address does not show up in your bitcoin client, just copy / paste it in the window for creating message signatures.
Thanks to jackjack-jj for providing a verification library.