Chromium* has undo. Fill out an entire form and you can undo, redo the entire form change-by-change. Not sure about the other browsers as I don't use them with any regularity

* Currently running Chromium 15.0.869.0 on Linux.

never noticed that.

did find someone who wrote some js to help:

http://www.andrewingram.net/articles/undo_form_reset_script_...