Inkscape

Screen: 
Inkscape screen dump.

Inkscape is an open source vector graphics editor, with capabilities similar to Adobe Illustrator using the W3C standard Scalable Vector Graphics (SVG) file format.

It also includes tracing functions.

Documentation

Tutorials

Problems

No CMYK mode

One problem is that Inkscape works in RGB because this is what the SVG file format can handle. This is not really true. As of version 0.48 Inkscape can work with colors in the CMYK color space. This means that you can assign an object a CMYK color using a given color profile and it will import correctly into Scribus (see Getting CMYK colors from Inkscape to Scribus for printing).