I've stumbled into these two quite a lot lately.

• http://simpsonizeme.com/ - a funny tool letting people convert their pictures into simpsons. Burger King, harnessing the wave of Simpsons-the Movie, created this promotion campaign.
• http://www.cokezero.dk/ - a tool letting users create lines in famous movies scenes, and see how it would be like. Created by CocaCola to promote their new product, Coke Zero.

Recipe: take an already well established brand (this kind of promotion works best for these), add a useful and not that trivial online tool that turns to the wide public (something fun and "cool"), and you get an interactive viral message. It spreads by itself, pretty fast and wide, it sticks for long, and it does the job very well. This is always the case with interactive advertising - one in which the user/consumer takes an active part - we don't push the message, it's just hovering over the user experience.

It worked on me :) Here I am in a simpson version (simpsonathan):

A friendofmine went as far as converting his whole family, turned out to be very funny - the Liebensimpsons