Krita is a free open source graphics editor for Windows, Mac and Linux. The software is geared towards digital painting and artists but it also has the features that enable it to be a fairly decent program for photo editing and general graphics editing as well. The software does support multiple and grouped layers, including vector layers and has a good selection of default tools and filters to meet most needs, but still falls short of Photoshop.
Krita does not support plugins yet, but you can import brushes, patterns and bundles, as well as record and import macros, and Python script support is on the way. The software can be used on both regular desktop computers as well as touchscreen displays.
The development of Krita is in part crowdfunded through Kickstarter and they actually allow supporters to vote on features that should be developed for Krita, which I think is pretty cool.
I have previously made a basic tutorial for photo editing with Krita, if you are a beginner check it out to learn the basics of photo and graphics editing with Krita.
Krita is good for general usage, but especially good for sketching and painting, and that’s where their future resources will likely be focused in terms of software development.
Multiple layer and vector layer support.
Designed for digital painting, sketching and drawing.
Tools and filters usable for photo retouching and general graphics editing as well.
Funded through Kickstarter, supporters can vote on new features to be developed.
Macro support available and Python scripting support coming soon.