How to capture my post request when I send a form to a server?

I face some problem with a form on a web server. To solve it I want to know what exactly gets sent, when I click the submit button. How can I sniff the packets and see the parameters who get sent? Is there a good tool for mac to sniff exactly this post request I am doing?

Thanks Markus

Solution

the simplest way is using a GET and see what's being sent via the browsers' query (altough the way you are sending could be AJAX and this method won't work).

Aditionally you can use Firefox's Firebug to see what's being sent.