I hate to knock a free application, but Free Ruler has two big problems:
1. It uses way more CPU cycles than it should have to, causing my fan to come on full blast.
2. The drop shadow sometimes makes it very difficult to see details on the screen you're trying to measure. (You can work around this by moving the ruler and using the cursor, but still, the drop shadow needs to go.)
I've used Free ruler for a couple of years now, and I'm very grateful to the author, but I've just tried out a program called PixelStick that I'm finding more useful. It's not as intuitive as Free Ruler, but it's worth taking the time to learn it.