Free PDF viewer allowing to measure in a document?

PDF XChange Viewer has a fairly comprehensive set of measurement tools that I find quite useful. It is, however, Windows-only, I think.

Here's a sample:

(from my comment to this answer)

Another suggestion is to use a screen ruler on top of your .pdf document. For Linux, I strongly suggest ScreenRuler:

ScreenRuler is a small GNOME based utility that allows you to measure objects on your desktop. It can be used to take both horizontal and vertical measurement in 6 different metrics: pixels, centimeters, inches, picas, points, and as a percentage of the ruler's length.

In Fedora, I know it's easily available through yum install screenruler. In Ubuntu is probably the same.

Consider the following example:

\documentclass{article}

\usepackage[top=1cm, left=3cm, right=2cm]{geometry}
\usepackage{lipsum}

\begin{document}
\lipsum
\end{document}

After compiling it, I opened the resulting .pdf file and ran screenruler:

As DevSolar mentioned in the comments, if you zoom your file, you need to manually set the dpi resolution. :-)

For Mac OSX users here are two options:

In Preview,

1. Select a rectangular area to measure. Either with the toolbar button or by selecting "Rectangular Selection" under the tool menu.
2. open the Inspector (Command-I)
3. Select the Crop and Rotate tab, whose icon looks like a small ruler. Change the units as required
4. Now use the select area tool and the corresponding (printed) dimensions will be shown in the Inspector.

-OR-

Download Free Ruler, which displays a nice ruler on screen, though requires care in both setting the ruler DPI and ensuring that the PDF is viewed at 100% size.