xScope from Icon Factory is a powerful set of tools that are ideal for measuring, aligning and inspecting on-screen graphics and layouts.
I purchased the program for OS X several weeks ago and I use it daily, infact, hourly. There are 6 different on-screen tools that help in my daily work, Rulers, Screens, Loupe, Guides, Frames and Crosshair. Icons for which can be seen in the second image, I have three of the icons visible in my menu bar and the Toolbar I have turned off.
The three I use are Rulers, for measuring the widths of things on screen in Pixels, this means I no longer have to take a screenshot, import into Photoshop, select the area and note the size. These rulers just overlay on my screen and I can extend them to measure anything underneath!
Screens is a nice tool, not used so much however, it simply overlays a box on the screen at a set size, for example 800×600, or 1024×768. A quick and easy way to check that a webpage fits into a smaller screen size without having to change the resolution of my computer.
Lastly, Loupe, which is invalueable. It dispalys an 8x zoomed in area of my screen around the mouse, it has a pixel grid and on hovering over individual pixels shows the HEX colour code, RGB colours and HSB, and also the X,Y co-ordinate of the mouse at the time. I use this all the time to quickly pull colour codes from something like photoshop into skEdit for placing in my CSS style sheets.