Kirito's Hameln, from his second solo live DVD ("Existence Proof").
...yes Kirito is a little out of key at the beginning, but it gets better afterwards!
(This was first ripped to be sent to someone who understands Japanese: sorry for the long speach at the beginning of the video.)

-If you like it, CD Japan, HMV and other Japanese online shops (who sell official versions, not bootlegs) will be delighted to send it to you.-