xScope is a great Mac app that makes it easy to measure on-screen dimensions with a wide range of tools. I use it frequently during the work day dealing with websites under development. Version 4 brings a ton of new features that I can’t wait to play with.
xScope 4 can be bought directly from Iconfactory or via the Mac App Store. The regular price will be $49.99, but is currently on sale for $24.99.