You probably know that it's easy to fiddle with Web form data and Web request headers. Intruders do it to find weak spots in your defenses, so maybe you should do it test your own systems before the bad guys do.
There's a nifty plugin for Firefox called Tamper Data that lets you manipulate Web headers along with GET and POST parameters. Head over to Mozilla's site and check it out. There are a few screenshots that help you get an idea of what it looks like in action.