On-screen Ruler 1.0.1 is a nice little on-screen pixel ruler with many features (available in one right-click). It will make your life easier when you’re transforming a psd to xhtml + css, designing a layout in photoshop or simply measuring something on the screen.
Publisher’s description of On-screen Ruler
This program is a semi-transparent on-screen ruler with keyboard shortcuts to help you lay out GUIs and Web pages or measure anything on your screen. Right-click to learn the options and shortcuts. The ruler saves its settings upon exit, so it always looks the way you configured it last. The program comes with a built in uninstaller. It is safe for work, since it does not modify your registry.