Here’s a common workflow: “I want to overplot a curve from the literature on my new plot. I could write the author and wait several days for them to dig up the plot file and send me the digitized version, but I want to compare now!” One solution is to digitize the published plot.
I used to use Dexter, but now I’m in love with GraphClick ($8, shareware.) Just screengrab the plot, paste it into GraphClick, click a few key points on the x and y axes and type in coordinates, and then either choose your data by hand, or use one of GraphClick’s curve-finding algorithms to automatically identify data. You can organize your digitized data into multiple datasets, which you can save as text files. Plus you can save the whole project, should you need to come back later and alter a fit.
Here’s an example from a recent paper (Mannucci et al. 2010, Fig. 6). I used the curve-finding algorithm to follow one of the curves; the digitized points are shown by little red dots. This is a fairly perverse case, as there are multiple overlapping curves; but it took less than a half-hour, start to finish, including send the output text files to my collaborator.