Inkscape. An open source vector editor
If you work with illustrations, you know it’s better to use vector editors, like Illustrator or Sketch, than to use bitmap editors like Photoshop or Gimp, because they give you much more flexibility and fine grained control on the final result. Inkscape is far from a replacement for Illustrator, but it is a simple yet powerful vector editor that can import Illustrator files, and saves in SVG format, something rather useful in the web of today. It has versions for Windows, Mac and Linux. Its development is not as fast paced as we would like, and it still lacks a number of features other editors have, but it’s perfectly usable as is if you keep your expectations in check.
Here’s a simple tutorial to get you started: