Inkscape supports many advanced SVG features (markers, clones, alpha blending, etc.) and great care is taken in designing a streamlined interface. It is very easy to edit nodes, perform complex path operations, trace bitmaps and much more. We also aim to maintain a thriving user and developer community by using open, community-oriented development.

because it is SVG compatible, you can display it in firefox which makes it very cross-platform. It doesn't have nearly as many stencils as Visio, though.