sometimes you have to hear someone else sing a song to realize it is pretty good. While there are ironic or snotty cover songs out there (see Ben Gibbard's Complicated), here are some great covers of pop songs by respected artists:
travis does Britney
ted Leo does Kelly
Clem Snide does Christina (my personal fave)- move past the talking at the beginning (to about 1:44)
What are some of your faves?