It depends on the result you want:

• For a bitmap image that looks like a sketch or cartoon, try GIMP. It has filters that can create drawing-like effects, and many basic photo editors offer similar stylized filters.
• For true vector tracing, try Potrace. It’s designed to convert bitmap images into vector paths.
• You can also try Autotrace, which uses a different tracing method than Potrace. Since the two tools work differently, one may produce better results than the other depending on the image.

If you want clean, scalable line art, vectorizing is usually the better route. If you just want the photo to look hand-drawn for screen or print, a bitmap filter in GIMP may be simpler.

There are also web-based services that can make line-drawing effects, but if you specifically want free downloadable software, GIMP plus Potrace/Autotrace are the main options mentioned here.