I saw a handy little favelet roll across the del.icio.us feed today:
By far the most complicated favelet I’ve attempted, this one fails in MacIE, but works fine in Camino/Chimera & Mozilla, so it’ll presumably be okay in other Gecko-based browsers.
It shows the mouse’s current position in the browser window, and allows you to measure distances via a draggable marquee.
Neat. I didn’t try it in IE, but I’ve read that it doesn’t fly due to IE shorter boorkmark URL size limit.
That also led me to the SlayerOffice favelet suite, which is sort of a 15-in-1 favelet swiss army knife for web development. It includes the ruler favelet, but that one didn’t seem to work right for me, so I listed it here separately.