While looking for something completely unrelated, I discovered Inkscape, an open-source vector graphics editor (something like Adobe Illustrator). Inkscape is Open Source (and thus free in all senses of the word) and available for many platforms, including Windows, Linux and Mac. In contrast with Sodipodi, the only other competing Linux package, Inkscape seems more mature and - most importantly - has great tutorials (really not common in the Open Source world).
My second - quite accidental - discovery was the fact that Firefox 1.0.7 seems to natively support SVG format quite well. For example if you click here, you can see vector penguin right in your browser window!
And while we are talking about vector graphics and Linux, this initiative also looks really promising. I tried Xara several years ago and it certainly looked very... interesting and fresh.