It's free.. you'll just get static if you try and record past 10 minutes of audio without registering.
(Most YouTube's are less than 10 minutes right? - you can always record 9.59 of sound then use audacity to splice the two tracks together... not the most elegant of solutions though!)
Try it, it's a great app, it's in my top 5 useful apps alongside... VLC, Jumpcut, Snapz and Dragster.
(I've happily registered all of them, long live the independants!)