A form that has a list of 2 items. If the user was to select a certain choice then a textfield and text area became available. I would prefer that these areas not seen until they choose the option that gernerated it, but I guess another option would be that these two additional fields would be disabled until that field was selected.
List: "Send To:"
Options: "Me" or "As a Gift"
If they chose "As a Gift" it would generate a "Name" textfield and "Message" textarea.
I did something similar to this on one of the custom e-commerce apps I built at work. Of course it was done before I knew anything about jQuery, so it’s a bit messier than the example Chris has provided, but the theory is the same.
I have the submit button deactive until the user fills out all the required fields and checks the box indicating they’ve read the liability waiver and agree to the terms (or in other words, poor mans digital signing of said waiver)
I’d worked on a similar thing several years ago for a clothing store site I was working on with a guy, we populated all the potential drop downs and whatnot from the database, and kept them hidden revealing them only based on what selections were made earlier in the form. The smarter way to do that these days would be ajax.