Hi Ton,

Take a look here:
http://www.artviper.eu/mootoolssecureform/
and here:
http://www.roscripts.com/AJAX_contact_form-144.html

Personally, I'm not too much of a fan of Ajax when it's not needed.
There's a contact form available in SCMS that uses solely PHP. You can copy/paste the form from the file frontend.php in the contact module folder.

User interaction is the subject here, people are used to press submit after they completed the form. So for a contact form I think it's not really necessary to use Ajax. And just because you can, it doesn't mean you should use a particular technique Is there any specific reason you need Ajax? If the goal is to let people contact the website owner, you are fine using the built-in contact module from Joomla.

On the other hand, for example when you make a registration page and the user has to pick a username, I think it really helps the user to tell them if the username is available or not before pressing submit. It depends on the situation and what you want to do with it.