http://navsurf.com/dhtml/acemenu/
Have a look at the above link.. I know you requested FREE but we've used this many times before whilst designing websites and have found it to be really good.
Infact I think you can get a free download if you don't mind their company name on the menubar somewhere otherwise i think it is about £35.00.
Hope this helps.
Roy D
www.optihost.co.uk