Working more and more with ASP.NET MVC I’ve found the Tamper Data Firefox Add-on to be quite useful. With this add-on you can view HTTP/HTTPS headers and browser-requests and (which is really awesome) edit POST parameters.
Use this add-on to security test your web applications by modifying POST parameters and simulate a scenario where a user changes/tampers with information sent to your site, like e-commerce sites where a user tries to buy cheap stuff, hack passwords, etc..