LightZone is a "digital darkroom" for your JPEG and RAW images. It's been around for years and was once a commercial product, but is now open source.
Getting started can be as easy as opening an image and choosing a "Style", a built-in combination of editing settings. Examples are "Soft Skin Tones", "SLR Sharpen", "Midtones Sharpening", "Skin Glow", "Bright Scene", "Dark Scene", "Fill Flash", "Crisp", "Hard", and "Soft Toning".
There are also multiple tools available which allow you to take more control, including "Relight", "Sharpen", "Gaussian Blur", "Hue/ Saturation", "Color Balance", "White Balance", "Black and White", "Noise Reduction", "Clone", "Spot" and "Red Eyes".
Most of these give you plenty of scope for fine-tuning. Click "Sharpen", say, and you're able to set an amount, radius, threshold, blending mode, opacity, make a colour selection, and more.
LightZone's "regions" are vector masks which restrict a tool's effect to specific areas of an image.
Whatever tools you use are organised in a "stack", similar to layers in other applications. This means that if you carry out several operations on an image, you can always step back to any of these, modify or delete it, and view the results right away.
Please note, the developer requires registration before you can download LightZone. This essentially means following the usual signup process for any web forum: click "Create new account" under the user login on the right hand side, choose a user name, enter an email address, respond to a confirmation email and set a password. After that download links appear for Windows, Linux and Mac.