Script Summary: [NO LONGER MAINTAINED] Allows you to run code from a Greasemonkey sandbox.
I know something like this can be a little unsafe, so I took some precautions. I was worried about, say, a page script finding the shell, placing malicious code inside the shell's textbox, then firing a fake click event on the "Run" button, but as it turns out, there's a way to detect faked click events, so AFAIK this is secure. (Faked click events always have their layerX property set to 0 -- you can't set it on a faked event.)
UPDATE 04-03-2009 (MDY): It is strongly recommended that you install this update.
Added all the missing features. You can now clear the listed results and enlarge and shrink the modal. Also followed a helpful suggestion from sizzlemctwizzle to prevent CSS on the page from messing with the shell. A fix was also implemented so you no longer have to worry about glitching the shell (in the previous version, the shell's own variables could be accessed from its command line).
Also added a neat feature: you can use the four-arrow symbol near the Shell's upper-right corner to click and drag the shell around.