They're all designed by different people. Though in a sense they all use me, since I nag them to fix the more egregious design mistakes.

They're lucky, and you're doing a great nagging job.

I think their secret is that YC deliberately tries to choose founders with a certain design sensibility.

I've noticed this, too.

Wufoo are famously good, and Kevin does all of that work. SocialMoth/Overhear.us have awesome design and Evan does everything. iminlikewithyou.com is tasty (if a bit overly pretty), and I believe they do it in-house, as well. Reddit is great and clean, and Alexis and Steve designed it.

Everyone is swayed by a nice clean design, even technically savvy folks like YC (they also know that a good design can make a good product great, and with bad design the best product will probably fail--MySpace being an obvious and baffling exception to that rule).