Firebug lets you change CSS and HTML and displays the result immediately on your screen. Sometimes with a strange display bug I'll go through and make a number of CSS changes until some combination of them have magically fixed my bug. This plugin will tell me everything that I've done so that I can make the necessary changes to the actual files.

The next step would be a "commit" feature that automagically persists your changes into your source code in exactly the same way you'd do it manually.

I'm getting a bit sick at the "read-only" nature of most source code.