PMeter

PMeter is a very handy pixel meter. There are plenty of desktop rulers around, but its hard to find a good one, which is free. PMeter is a simple to use dektop ruler allowing to switch between 3 user configurations easily. For each configuration you can set width and colors individually, choose to add a desktop magnifier to your ruler, and more.
PMeter also includes a color picker in case you need to know or work with the color of any pixel visible on your screen. The additional display for mouse cursor coordinates might also be useful for you.

Of course PMeter is a portable application you can carry with you on your USB drive, but it can also be installed locally to your hard drive.

Using PMeter is simple. However its good to know a couple of things...

double click the ruler to rotate it by 90° (also available in the rulers context menu)
hold down the shift key to move the ruler horizontally or vertically only
to pick the color of any pixel click at the color picker box and drag to mouse cursor while holding the left mouse button