A ruler.

PS: Did anybody already mention screenlets.org? I guess this could be a VERY nice upstream source, because according to the linked site:
* Easy to use, easy to develop
* Hundreds of Screenlets available
* Works with any composited X desktop (compiz, xfce4, ...)
* Included themeing-abilities (SVG, PNG or mixed)
* Fully scalable when using SVGs
* Embedded drag&drop-support
* Automated storing of options (using ini or gconf)
* Controllable through customizable DBus-service
* Free, open-source and released under the GPL
* Uses cairo and GTK2 for drawing and windowing