add pwn to the beginning of the URL.
for example, you want to download this:
http://www.youtube.com/watch?v=VpmOTGungnA
add pwn, so the URL becomes
http://
pwnyoutube.com/watch?v=VpmOTGungnA
To download video, I use the KeepVid link.
To download mp3, Online-Convert is reliable.