A few months ago I published a quick comparison of 3 opensource cmses that come with a form builder. At the time the form builder feature was quite new and innovative, it now comes as a standard feature in a lot of web publishing solutions. I have reviewed a few hosted form makers, such as
Wufoo (hosted),
MachForm onsite application available for less than 100 dollars,
FormAssembly hosted (on site version available for a few thousand dollars),
FormBuilderPro (hosted and onsite application)
Experimental Free opensource jquery form builder
[2009-10-23 edit] Sonoku's Form Builder
They all have very nice interfaces including data gathering and statistics but the one I loved best was MachForm that is also distributed as a standalone application fro a very reasonable price. It uses Jquery as the main javascript library and lets the user build forms using drag and drop. One good idea is the composite field that lets the user add adresse field in one click. The screenshot below displays my own translation of the admin interface to french.