Inkscape is an open source SVG-based vector graphics editor that comes with a host of high-end, professional features.
You get a lengthy list of drawing tools, for instance, quick ways to create rectangles and squares, circles and arcs, spirals, polygons, 3D boxes, Bezier curves, straight lines and more. But there are also interesting sculpting and painting options, a lengthy list of path creation and editing options, and some very powerful text tools.
In common with other SVG editors, you're able to scale objects in a click or two, group and ungroup them, apply colours, gradients and strokes as required, align and combine objects, trace bitmaps and more.
And there's direct support for many advanced SVG features, like alpha blending, markers and clones.
There is a price to pay for this, as the learning curve is a little steep: you'll need to invest plenty of time and effort before you've properly mastered everything that Inkscape can do. If you need a good vector editor then it's well worth the effort, though, as Inkscape provides many of the same features as Illustrator for no cost at all.