Free and open source (GPL3) photo processor.
Photivo handles your RAW files as well as your bitmap files (TIFF, JPEG, BMP, PNG and many more) in a non-destructive 16 bit processing pipe with gimp workflow integration and batch mode. Read more.
Alternative apps to Photivo:
An easy-to-use image and photo manipulation application.
Create, convert, edit and compose images
Photoshop CS2 pushes the envelope with powerful features and simplified workflows that provide photographers and creative professionals the freedom to deliver stunning images.
Speedy Painter is an OpenGL based painting software for Windows. It supports Wacom digitizers to vary size and opacity of brush strokes according to pen pressure.