Engauge Digitizer - This open source, digitizing software converts an image file showing a graph or map, into numbers.
WebPlotDigitizer - WebPlotDigitizer - Web based tool to extract numerical data from plots, images and maps.
Plot Digitizer - All-in-One Tool to Extract Data from Graphs, Plots & Images
DataThief III - DataThief III is a program to extract (reverse engineer) data points from a graph.
im2graph - im2graph graph digitizing software to convert graphs to numbers
Silkscientific UN-SCAN-IT - Digitizing Software converts graph image files to usable (x,y) data automatically