What's the best way to do this, using free software?
I want to convert photos to line-drawings, maybe cartoons ...
Do you want a bitmap image that looks like a drawing or do you want to vectorize the image?
To create a drawing effect, there are several filters for GIMP that will do that into some extent. Many simple photo manipulation programs have filters like that, too.
For vectorizing your image, you may want to try potrace.
In addition to potrace/autotrace, you might want to look at autotrace. This uses a different approach than potrace, which traces the outlines of polygons. Autotrace appears to be a kinda dead project, but it's worth trying in addition to potrace, because they both give different results and one approach may work better with your image than the other.
All of this software is already packaged for Fedora or Ubuntu, so it's just a
apt-get install away.
Bit of an old post now but I was just looking around and found photolinedrawing. I did a couple of holiday snaps on it and they came out fine; one had some funky waves on a beach shot and came out a little strange when it converted to a line drawing but looks kind of cool as its different. Got a couple of free credits when I registered so didn't cost me anything. Its not software as its on the web so might not be useful if you want software you can download but worked for me.