Further to mindbullets reply, if you're talking about the C4 episode, it is Mad World (originally by Tears for Fears), but performed by Gary Jules and Michael Andrews. If you mean the E4 episode, this is a good site for Smallville music info:
http://www.kryptonsite.com/
. I'm way behind on Smallville, so I don't know if an REM like song was at the end of the E4 episode, but when I heard the version of "Mad World" mindbullets mentions on Donnie Darko, I did think it was by REM!