Inkscape has come a long way. It’s still got some problems, as evidenced by the fact it isn’t at v1.0 yet. I feel it’s in than some of the others I’ve tried like sodipodi, etc.
It is cross-platform, free, and if you’ve never used a vector art program, the techniques you learn will apply to any other vector program, which is a very good skill.
If you are using Illustrator or something like that at school, there are some differences that may be a bit frustrating, but it is free after all.
Every so often, different versions of Canvas have appeared on some of the FuturePublication magazine+disc combos (Digit, PCPlus, etc). The latest I have is version 8, and I like it a bit better than Inkscape, though still crash-prone. If Inkscape had been at its current level before I got Canvas, I might not have bothered learning Canvas though.