DigitizeIt

DigitizeIt is a handy tool to extract data from charts in any popular image format, such as JPG, PNG, BMP, and GIF.

To use it, just upload the image you want to analyze, set the X and Y axes, assign the values manually, and finally, process and extrapolate the data to an ASCII document or in scientific notation.

DigitizeIt requires some mathematical knowledge and a bit of patience, since its interface is not quite as intuitive as one might wish and may seem overwhelming at first. Still, there aren't many other programs like this, so it's a useful tool.