The one I can recommend can be found if you google for 'yaromat' - (mods: I have no affiliation with him).
Although JS form checking is good I would also advise to use server side checking (if you use a script to email you the form).
If you use matts script - again no affiliation ;-) - or any of its variations, you have a built-in facility fo check for required/mandatory fileds using the <input type="hidden" name="required" value="yourmandatoryfieds, separated, bycomas" />
Hope this helps
[edited to fix some typos]
[edited by: le_gber at 10:33 am (utc) on Dec. 2, 2005]