There are many, many more security testing proxy tools, this is just a short list of a few of the quick, easy, and nice tools to use.
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It can be used not only to change form input tags, but also the cookie's that are currently set in the browser, and any other value within a website or web application.
Additionally validate the input every time, not just when the data is initially accepted.
For example if you set a cookie, make sure that cookie is the same value and it is correct on each and every request.
You can do this with a number of different applications.
I've included a list of some of the proxy applications that allow you to do this.