Raw Therapee is an open source image processing tool which provides a huge range of ways to enhance your digital photos and other images. (As the name suggests, it's intended for use with raw files from digital cameras, but the program can also import JPEG, TIFF and PNG files.)
The vast array of high-end novices will quickly confuse the image processing novice, but if you know what you're doing then there's a great deal to explore here.
RAW image quality may be enhanced by a good range of demosaicing algorithms, for instance: AMaZE, DCB, fast, AHD, EAHD, HPHD & VNG4.
Colour handling controls cover everything from simple channel mixing and RGB curves, to white balance, HSV, vibrance and more.
Would you like to enhance details? The Detail pane contains no less than seven options: Sharpening, Edges, Microcontrast, Impulse Noise Reduction, Noise Reduction, Defringe and Contrast by detail levels. And each of these is highly configurable, so for example select Sharpening and you can choose the algorithm (Unsharp Mask or RL Deconvolution), radius, amount, threshold and more.
And you also get great exposure controls, a stack of powerful lens/ geometry correction tools, EXIF/ IPTC metadata editing, batch processing, command line control, lots of configuration options, and the list goes on.
Beginners should look elsewhere, then, but if you're familiar with this kind of high-end image processing tool then you'll get a lot out of Raw Therapee. Its interface is occasionally clunky and performance isn't always as good as the authors appear to think, but there's an enormous array of processing power on offer and it's a great way to get the very most from your digital images.