A hip program that I found useful recently is an app called Delineate.
As it works with Java, it is completely cross-platform and runs using client executables of autotrace or potrace.
The great thing about it is that you can play with the settings of either program and then generate a preview. Two images are shown so that you can easily compare effects side-by-side.
Once done, you have a nice SVG ready to throw into Inkscape to play with.
Check it out.