Pricy, but better than you think it is...
I've been looking for alternatives to this for years now, but finally bit the bullet and purchased xScope. NO regrets -- and now that I know I'll keep it around, I find I use it much more than I actually thought I would. The Guides and Dimensions panels save SOOOO much time when laying out UI components.
My only gripe is the annoyance of its always-present Dock and Task Switcher icons. I prefer system-wide utilities to sit quietly in the background until I conjure them via keyboard shortcut or menubar icon; otherwise, it's yet another thing competing for my attention on a developer's already-crowded Dock . Not sure why IconFactory developers insist that this is "the only way to do it," when Apple clearly documents how to make menubar-based applications that don't require the Dock icon (a la Annotate/Glui, for instance).