xScope ($27) is a great utility for Mac-based web and UX (user experience) designers. It provides a number of floating tools for measuring, aligning, and inspecting on-screen graphics.
You have to play with xScope to get a sense of how helpful it is. It has on-screen rulers, floating guides, and a nicely designed screen overlay that shows the usable space for any screen (including new templates for the iPhone). My favorite feature is a new dimensions tool (shown above), which instantly identifies the pixel dimensions of anything on your screen. Link to the movie below for a better sense of how cool this feature is:
Download a free trial of xScope and give it a spin.