Inkscape is quite wonderful. Most of my interest in it has to do with its SVG capabilities. Nearly everything useful I know I've learned from Tavmjohn Bah's book, Inkscape: Guide to a Vector Drawing Program. Tutorials and related introductory materials include:
Phil Shapiro is a persistent cheerleader for Inkscape.
Other Inkscape-related items:
My colleague Phil Shapiro asks, "Can Inkscape reduce the number of incarcerated people?"
[Much more to come.]