Krita is a free and open source graphics editor for Windows, Mac OS and Linux. It is designed primarily for graphic artists for digital painting and 2D animation, but it is also a competent editor for photos. Its user interface has the usual features with multiple configurable tool panels and straightforward menus. Workspaces can be customised to suit your workflow.
Krita supports layers and masks, blending modes, a wealth of non-destructive filters (with real-time preview), different colour models (RGB, CYMK, Lab etc, and so on. It supports at least some RAW file formats, but lacks a sophisticated RAW processing back end (ie no equivalent of Adobe Camera Raw). If you want, however, you can process your RAW files in powerful free programs like DarkTable or RawTherapee (or Adobe Camera Raw – also a free download at present), then export to TIFF or PSD to import into Krita for further processing.