A simple test for the Wacom web plugin. Docs are here and here. See also a basic test application, a scribble example), and a more advanced demo example.
Wacom tablets are interesting input devices: they provide sub-pixel resolution input (more precise than screen pixels), information about the stylus pressure, proximity data (the pen is sensed even if it does not touch the screen) and device identification (e.g. pen tip vs. eraser tip).
Unfortunately, in order to receive such data from a Web application, a plugin needs to be installed (download from here if you feel lost in the messy Wacom website). You also need a Wacom driver for your tablet, not the one Windows installs automatically (I downloaded mine from here, but there's the chance you need a different driver, for example if you have a professional tablet like Cintiq or Intuos). All of the above works under Windows, I don't know about other OSes. If you make it work in other platforms, you can drop a line in the comment section of this very Gist.
See also my desktop application for reading stylus data, written in Python.
the links in this appear to be broken