Plugin to add client side JS forms validation ?

Yes. But people appear to have a not so easy time using it...

http://www.agilewebdevelopment.com/plugins/client_side_validation
http://www.agilewebdevelopment.com/plugins/validation_reflection

Blame the author (me), better still, provide me with specific bug
reports and improvements. Also, read the README and bear in mind that
english is a locale, too (no, localization does not only refer to all
those other funky places out there).

Otherwise, I hope this stuff just works for you.

Michael

I was working on an AJAX remote model validator - I never officially
released it or finished it but it was sort of working. You're welcome
to checkout and play with the code:

http://opensource.agileevolved.com/svn/root/rails_plugins/remote_model_validation/trunk/