GIMP. Open Source graphics editor
In a world where Photoshop is a verb, everybody is aware of the power of a graphics editor. GIMP may not be a complete replacement for Photoshop, but is a very powerful and useful graphics editor that is free and open source. Its development is not as fast paced as we would like, but it slowly keeps getting new features. It has Windows, Mac and Linux versions.
Being one of the most veteran open source projects still in development, there’s lots of tutorials online, but most of them are outdated. Here’s a simple and recent introduction: