I really like the icon for the Firefox add-in Greasemonkey. You can disable it or enable it. Guess which state each is?
I love that it is fun - the monkey is sad when disabled. It makes me want to enable it.
BTW, Greasemonkey is an add-in for Firefox that allows you to customize pages. For example, you can change the logo on the Google Search page, or add a button to Blogger (that's what I was using it for - until I found Windows Live Writer which works even better).
For more samples of Greasemonkey at work, see the Wikipedia entry: http://en.wikipedia.org/wiki/Greasemonkey
For starting to write your own Greasemonkey scripts (if you know Javascript, it's easy), Mark Pilgrim has a great book: http://diveintogreasemonkey.org/ (online with samples, as well as dead-tree books on the subject).
