There are a bunch of other form builders out there like FormStack[1], Zoho[2], etc. I haven't used those 2 personally (only Wufoo) but I'd imagine some of them have some more advanced functionality with their more advanced plans.
Thanks for sharing. I've briefly examined the features of both services as well as other similar ones, but they all mostly focus on the front-end side and tightly integrate with the back end. I'm sure it's possible to hack together your own version of the font-end. However, that still wouldn't let me, for example, limit the number of sign ups for a certain event.