Engauge

Engauge is a free software which is useful to pick out data points from a published graph / figure. This is particularly useful while trying to superpose older published data with new results, when the original data files are not readily available. The software interface is pretty straighforward and the help files are decently elaborate. It also has an automated utility to try and detect data points or you can go the manual way by clicking each point that you need. All in all, a very useful utility for technical writing.

Update : I also found a few other alternatives, but I have not used / tested them. Here they are in any case - Plot Digitizer and g3data (this works only for Fedora OS).