PixelWindow has been discontinued. Similar apps include PMeter, which is free. Other options are On-Screen Ruler and Ruler. In total there are three apps like PixelWindow for Windows.
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.
