Since the Deet went first on the ol' pimpin'....I'll hawk a few of my own works. I have an upper-body fascination FAR more than one for feet, and the belly/ribs/hips just happen to be my favorite parts of the female form to torment...and it shows in my art.
All of the Of Inhuman Bondage series of pubs are heavy into belly tickling. Also, check out The Chronicles of Nadya and Sara's Tickling Fantasies. The latter is an older pub, and the artwork is not up to the level of the others, but the tickling intesity is still one of my personal faves that I've done.
Maybe I'll follow suit with the movie industry and do a modern re-make of that one at some point....hhmmm....
I'll also agree on Scavenger and Bandito's work, very sexy and hot belly tickling from these two masters.