Inkscape is an attempt to be to Illustrator what The GIMP is to Photoshop, only not horrendously ugly and hard to use.
Like The GIMP it's an X11 program, making it somewhat unpleasant to install and distinctly non-native.
Inkscape (2d drawing)
2d (Vector) Drawing
Free
Fairly Easy to Use
Powerful
Stable
Open Source
Standards Compliant
Mac | Windows | Linux