I may be rather late to the party but I recently discovered Iconfactory's xScope last week while graphic designer Justin Ruckman was in town working on the next version of Skribit. I saw him using xScope's Dimensions tool to painlessly measure distances between on-screen elements. I was hooked from then and started using the xScope trial. In the last week xScope has proved so immensely useful that I had no hesitation paying the full 26.95 to buy it. Let me show you why.
The xScope menu mapped to its handy menu bar icons.
xScope is comprised of 7 different tools and the biggest difference between it and other measurement tool suites (commonly just popular Firefox add-ons) is that xScope works for anything on-screen. It doesn't matter if you're working with some elements in OS X itself, or within Firefox, Photoshop or what have you. It just works as an overlay.
Why is this useful? While a user might have say a 1024x768 screen resolution, browser and OS tool bars take up space. Screens makes it easy to find out what your above the fold real estate looks like.
Pro Tip: Hit Command+D to freeze loupe's selection. Tap Shift+Command+C to copy the currently selected HEX color to your clipboard (extremely convenient).
This screenshot is a bit overkill but shows off what can be done with guides. Each individual guide can be moved as you please.
Pro Tip: Hit Command+G to use the Guide Wizard and create a grid.
Mix & MatchWith the exception of the dimensions and crosshair tools, any of the xScope tools mentioned above can be used together. Popular combinations for myself include setting up guides and using the ruler as bounds for the dimensions tool.
OverallxScope is definitely worth its price. As for complementary applications, Justin Ruckman pointed out that he uses Color Schemer (now offline) for Loupe-like tasks. I give xScope 10 out of 10 Stammys.
Do you use xScope or anything similar for your graphic and web design needs? What are your must-have design apps?