PixelWindow is described as 'simple, light-weight, cross-platform application for measuring things on your screen. It creates a transparent window that acts like a pixel ruler with its width and height reported in the center. Just drag it around and resize it to measure whatever you want'. There are three alternatives to PixelWindow for Windows. The best alternative is PMeter, which is free. Other great apps like PixelWindow are On-Screen Ruler (Free) and Ruler (Free).
PMeter is a very handy pixel meter. There are plenty of desktop rulers around, but it's hard to find a good one, which is free. PMeter is a simple to use desktop ruler allowing to switch between 3 user configurations easily.
On-screen Ruler is a lightweight, free, semi-transparent ruler to measure portions of the screen. It has a lot of features (and related keys shortcuts) and several language translations are available too.