Inkscape is an open source vector graphics editor similar to Adobe Illustrator, Corel Draw, Freehand, or Xara X. What sets Inkscape apart is its use of Scalable Vector Graphics (SVG), an open XML-based W3C standard, as the native format.
Look at the image (screenshot from Inkskape site) and tell me if this application has not matured. If this is what version 0.45 can do, can you imagine what version 1.0 can achieve – for free?
Anyway, to give you more insight about this awesome application, Linux.com has a review of version 0.45.
For the stubborn readers who still refuse to switch from Windows, yes, Inkskape provide Windows binaries as well.