I wrote up the form builder WuFoo from the Office 2.0 conference. It's a fantastic tool if you need to put a feedback form on your site or collect any data over the Web. But there's another form tool that you might also consider: Icebrrg.
Here's the weird thing about Icebrrg, though, it's so similar to WuFoo that at first I thought it was a rebranded version of it. The form builder is nearly identical. The paid options are almost exactly the same. What's different? The Icebrrg interface has some rough spots where it gunks up the page a bit during form editing. On the plus side, Icebrrg can automatically send e-mail confirmations to people when they fill out a form. That's nice, but that's it. (A WuFoo developer says that feature is just about to roll out for WuFoo.)
If you need a form filler, my recommendation is to give your business to the company that broke the ground in the field. That would be WuFoo. I would not hesitate to recommend Icebrrg if it offered important new features or had better ease of use. But until Icebrrg starts to compete with WuFoo (and does more than add one fillip to the feature list), I'd say, stick with the original--just on principle.