Using WebSly, you can manipulate HTTP GET / POST based forms of your dynamic website. It is useful to test different scenarios where users may act in an "unusual" matter and post unexpected values to the server.

Although this project appears to be obsolete for Internet Explorer users since Microsoft has published the IE Developer Toolbar, it is still a nice alternative tool. It is also a useful resource for hobbyist programmers wondering how to manipulate HTTP forms programmatically.