Itunes will do it.
It has a built in converter. You have to make sure it's set to mp3, (file/preferences) then you just drag your song in,
right click on it, and select 'convert to mp3'.
It will leave your original file, and make an mp3 copy.
Once done, you can just drag the copy straight from your playlist to the desktop or wherever to save you searching for it.
Takes seconds!
Hope that helps.
foobar2000 all the way, its the only converter i ever need
I read the LAME encoder was good too.