SiteSpinner V2.7 and Web Engine (beta) have a Form Editor built in. On the top tool bar, go to Help > Tutorials > Forms for help. There is also an example form in the Templates folder.
An separate tool is WebFormDesigner that lets you easily create advanced Forms for your site. They even include instructions on how to integrate the forms with SiteSpinner/Web Engine.
Of course, you can always add Form Code to a SiteSpinner or Web Engine page by pasting the code into the Code Editor.
If you encounter any difficulties post a note to the '...how do I' form for help.