Quick links: TopTools is probably the best free one, and Screen Calipers are probably the best if you are willing to pay. If you just need to measure something in a web page, there is a Firefox plugin called MeasureIt that should do the trick for you.
Since this post seems to draw a fair amount of traffic to the site, I’ll elaborate on it. There is a piece of software out there called Screen Calipers, by Iconico Software. I tried it, and it’s rather nice. On my computer, moving and resizing the calipers caused flickers in the calipers, but that is certainly bearable. The Screen Calipers are not free, but they are probably the best for what they do. You can use them for an unlimited amount of time, but you are supposed to pay 19.95 for a single license. If you are really serious about measuring screen pixels, it’s probably worth it. I don’t think you will find anything better for a cheaper price anywhere. It is extremely well thought-out software. The interface is skinnable, and that’s pretty neat :). Screen Calipers by Iconico software is probably not for you if you don’t plan to pay for the registration; for one, the unlock reminders are annoying. If you don’t wanna pay (like myself), read on.
In search of a really really great screen pixel ruler, I found a lot of crap on the way. Cool Ruler is the one that I had used in the past. I guess it’s not too bad, but seems like there ought to be more out there. From the search results, it appears that JR Screen Ruler is a popular one. Too me, it’s too simple and ugly, might as well stick to Cool Ruler.
TopTools is my favorite free one. The memory footprint doesn’t seem too bad, and it’s got some neat features. Of course, there is room for improvement, but nothing is perfect. It would be nice to be able to adjust the size of the ruler by dragging the edge, and I can see where the screen docking could pose a problem if one needs to measure near the edge of the screen. Besides, there is a “slide to zero option” which pretty much does the same thing as docking to the edge of the screen. It also has a screen loupe (magnifier) which works in conjunction with the ruler. Seems like all screen ruler software should have that feature. I’d still like to find another ruler, though, there still ought to be a better one.
There is a program called MWSnap that does more than just measure units on the screen. In fact, that’s not its main purpose. Its main purpose is to be a screen capture utility, and it’s good at it. It also happens to have a screen zoom tool (loupe), a color picker (eyedropper, but don’t get MWSnap for that, get ColorCop instead), a window spy (mostly useless), and yes, a screen ruler!
MeasureIt is a plugin for Firefox that measures a square area. It could be used as a pixel ruler, and I use it for a pixel ruler myself. Just read the width or the height, depending on whether you need to measure horizontally or vertically. It’s mostly a convenience thing, since it’s right there in the browser.