Jonathan Coulton

Jonathan Coulton is one of the funniest songwriters I've heard in a long time.  He wrote Still Alive, the end theme to Portal, as well as the theme to Code Monkeys.  He is also featured on the audio version of "The Areas of my Expertise", as John Hodgeman's troubadour. 

Check out his web site, he's got free versions of quite a few of his songs, and you can buy his albums online.  I especially like "Skymall".  If you are planning on buying any of his music, they each song is $1, albums are about $10, and it is all DRM free.  You can get his stuff on iTunes, but if you go through his web site, more of that money goes to him.