I tried The Form Assembly and WuFoo a while back and liked both of them quite a bit. I ended up going with WuFoo to make use of their high-level form-elements (things like "address" instead of having to put in name, street line 1, street line 2, etc) and because the ajax interface seemed a little more intutitive to me.

I think both are well worth a look though!

You should take a look at Nenest Form Server (http://www.nenest.com), which is much more powerful to collection a variety of data.