GIMP is a cross-platform image editor available for GNU/Linux, OS X, Windows and more operating systems. It is free software. GIMP provides sophisticated image editing tools similar to those in Photoshop. There is a vast array of 3rd party plugins that can extend the functionality of GIMP.
GIMP is extensively documented online, including a range of tutorials ranging from then basics of installing and first use, through to sophisticated techniques like luminosity masks. The built in help module is extensive.
The following list (from the Help File) is a short overview of some of the features and capabilities which GIMP offers you:
- A full suite of painting tools including brushes, a pencil, an airbrush, cloning, etc.
- Tile-based memory management, so image size is limited only by available disk space
- Sub-pixel sampling for all paint tools for high-quality anti-aliasing
- Full Alpha channel support for working with transparency
- Layers and channels
- A procedural database for calling internal GIMP functions from external programs, such as Script-Fu
- Advanced scripting capabilities
- Multiple undo/redo (limited only by disk space)
- Transformation tools including rotate, scale, shear and flip
- Support for a wide range of file formats, including GIF, JPEG, PNG, XPM, TIFF, TGA, MPEG, PS, PDF, PCX, BMP and many others
- Selection tools, including rectangle, ellipse, free, fuzzy, bezier and intelligent scissors
- Plug-ins that allow for the easy addition of new file formats and new effect filters.
Gimp handles RAW files through use of a plugin like Darktable or UFRaw. Once these are installed, Raw file handling is straightforward.
Gimp supports layers and masks, sophisticated selection tools (some built in, some obtainable as plugins), and flexible file output options.
It is not as fast, and not as refined as Photoshop, but for the price ($0 for a perpetual licence including lifetime updates) it is superb. I use it sometimes for features that Photoshop lacks.