I listed Scribd more for the fact that their usability is really on point. I especially like the upload box on the front page. I'm glad to see more hacker types are putting time into making the user experience really good.
Also, I'd seen Particle Tree before. Definitely well-designed.
Wufoo as far as I know was designed by one of their team members who came from a design/usability background. They started a magazine called ParticleTree(http://particletree.com/) before launching Wufoo.
-Zaid