[HowTo] measure pixels on the screen

I guess that everyone has some tool to measure pixels somehow on the screen.

But since Sikuli claims to be the same on Windows, Mac and Linux, this little tool I found accidentally might be interesting, since it also claims to be exactly the same on Windows, Mac and Linux (based on Adobe Air).

... and it has some really nice features:
http://www.pixelwindowapp.com/

quoted from the page:
>>>
PixelWindow is a 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.
<<<

Tool for Windows only:
Point Position (http://www.snapfiles.com/get/pointpos.html)