Like a lot of web application developers I develop and test against Firefox first and IE second. I love a number of plug-in's including Firebug and Web Developer.
The Web Developer plug-in has some nice features such as the ability to view all form objects. Recently I discovered that with some simple CSS you can hide a form and its elements from the Web Developer display. Note, any user can still view source and see the form element, but it is still handy if you are concerned about site security. As it happens, I ran into this while working on some security enhancements for one of my clients.
<input type="hidden" name="username" id="fpUsername" value="" />
Now using the Web Developer toolbar you go to "Forms > Display Form Details" then the form would become visible. But you can hide it from this view by setting the CSS Display property to NONE.
While this doesn't really increase your sites security any, it is an interesting effect.