For photographers, a photo editing tool is an essential tool to adjust the color and tone of the digital photos that they captured. They can adjust the white balance, Hue-saturation-value, color toning and tone mapping of the photograph. You can non destructively (original image is never modified) edit your photos and can search your image collections by tags, image rating, color labels, and based on other filters. You can also crop, rotate and correct the perspective of your image. Here are some free, open-source tools that you can use to modify your photos without spending huge money on purchasing commercial tools.
Rawtherapee uses a 96-bit processing engine to get most details and least artifacts from the RAW photos. It also offers advanced color handling from white balance to Hue-Saturation-Value (HSV) curves, color toning, and color management. It can use different denoising methods like luminescence, chrominance, impulse and noise reduction. Parallel editing of multiple images in separate editor tabs. It can save JPEG, PNG, and TIFF images.
Darktable offers non-destructive editing means your original images are never modified. You can filter and sort your image collections by tags, image rating, color labels, and other filters. You can easily adjust contrast, brightness, and saturation. Adjust images by lightening shadows and darkening highlights. You can control the exposure by using the sliders in the module or just by dragging the histogram around.
Lightzone is a free photo editing software to edit your digital photos to achieve professional-quality results. It has a ZoneMapper tool to adjust the brightness and contrast of a photo for any combination of the highlights, mid-tones, or shadows. LightZone can open and edit raw files from almost all DSLR cameras.
PhotoFiltre is a complete image retouching program. It allows you to do simple to advanced adjustments to an image and apply different filters on it. PhotoFiltre has a layer manager, advanced brushes, batch module, and other powerful tools to edit and retouch your photos. You can do standard adjustments like brightness, contrast, dyed, saturation, gamma correction and also artistic filters like watercolor, pastels, Indian ink, pointillism, and puzzle effect.
DigiKam is an advanced digital photo management tool to manage large numbers of digital photographs in Albums. You can organize these photographs for easy retrieval using tags, keywords, captions, collections, dates, geolocation, and searches.
Irfanview is a graphics viewer application, but it can do some basic photo editing features like lossless JPG rotation, crop and EXIF date change in batch mode, slideshow, change color depth and others.