Fantastic free alternative to the flashier errr... non-Flash alternatives like Xscope. Especially for those of us who aren't full time designers but need an on-screen ruler from time to time.

It also has an especially pretty and intuitive interface for an AIR app, and quite a few extras like skins and device screensize pre-sets that even the more expensive alternatives don't offer.

Speaking of pretty, I know it's cool to hate on Flash: it introduces performance and accessibility issues, etc., but this developer's site design is fantastic.