Photivo is an extremely powerful open source photo processing tool which comes packed with advanced algorithms to help enhance your images.
And we really do mean "packed". If you'd like to sharpen a photo, for instance, the program supports lots of options: Gradient Sharpen, Wiener, Inverse Diffusion, Unsharp Mask, and more. And clicking the "RGB" button reveals no less than 16 ways to tweak your image brightness, colours, contrast and more.
The program also offers excellent tools covering geometry correction, denoising, demosaicing, vignetting, and the list goes on.
And most of these options are extremely configurable. Click "Gradient Sharpen", say, and you get to set values for Strength, Microcontrast, Halo control, Weight, Clean up and the number of passes. And other algorithms offer just as much fine control.
Photivo isn't aimed at photo processing novices, though, so if you're a beginner then you'll probably find all the settings a little confusing (initially, anyway).
You don't have to understand every single detail to get something from the program, though. And with a little time spent studying the online manual you should soon be getting more from Photivo and your digital photos.