JavaScript Shell

The JavaScript Shell lets you interactively run JavaScript commands, even against a particular window. This allows you to easily test out DOM manipulations you’re considering or, as the authors suggest, GreaseMonkey scripts.

Very nice, but it does not appear to be Safari compatible at all. Just goes to show that Firefox is far and away the best browser for development. I have found that Firefox’s tools are the easiest for getting things working, and then I deal with making things cross-browser.