Here is a new service that allows you to annotate the blockchain addresses that you own, or the transactions that you made.
link : http://ecdsa.org/annotate
Only you can annotate your addresses and transactions, because annotations are signed with your private key.
For example, if you donate to a charity of fundraiser, you can now add a message showing who you are.
This service uses the new "signmessage", "verifymessage" commands of bitcoind, available in bitcoin 0.5rc1 (https://bitcointalk.org/index.php?topic=47586.0