The prefixprompt script is a small tool I wrote that allows quick access to the Special:Prefixindex page, with an automated lookup. Clicking the link inserted in the toolbox opens a prompt in which you can type a prefix; the current full page title is included as the default. To look up pages prefixed with the current page title (useful for user subpages, etc.), just click the link then hit Enter (or click "OK"). To look up a different prefix, type it in the box and hit Enter (or click "OK").
I developed this script for use on my own (intranet) wiki, and have brought it here because it's just so useful to me.
Usage
editTo use the script, make sure you have the addLink() helper function available. Then all you have to do is add the line:
importScript('User:Voyagerfan5761/prefixprompt.js');
to your monobook.js. I have not tested the script with other skins, so try it at your own risk.