Here’s a couple sentence summary with links to many of the tools out there as alternatives to Adobe Photoshop Lightroom. This isn’t a complete list of all the Free Open-Source Software photo editing tools out there – it’s a list of tools I’m using, or have used in the past and found useful. I’ve highlighted the ones I couldn’t live without.

If you truly want to live the FOSS life, I suggest using a Linux operating system. I’m currently using Ubuntu, but there are plenty of others (Fedora, Red Hat, Mint are among many others). With Ubuntu, it’s easy to install as a dual-boot, so you choose which operating system to use on start-up and keep Windows or OSX and there’s a huge support community.

    • Raster graphics/image editing suite

        GIMP: current version GIMP 2.8.10
        GIMP is the closest thing out there to Adobe Photoshop. Most Photoshop users out there don’t like it because the interface is not the same and GIMP developers really aren’t interested in replicating it. But, it’s powerful, has an incredible number of people writing scripts and plug-ins for it, and it’s hard to find something it can’t do. Currently it doesn’t support 16 bit depth, unless you want to install the development version (2.9). Version 2.10 will be the new release, supporting 16 and 32 bit depth images. The interface has grown easy to use over the years, 2.8 has been really a lot of fun for me. I have little doubt that GIMP will end up a contender to Adobe in the future.
      Raw Developers

        Darktable: current version 2.2
        I rarely use anything else anymore. Similar to Lightroom – incredible powerful – the newest version (1.4 at this writing) allows for drawn and parametric masks. Takes a little bit of getting used to the interface, but worth it. Only available on Linux, Solaris, OsX. NOT available for Windows. If you are dead-set with the Windows OS I’d highly suggest you use Raw Therapee (next).
        Raw Therapee: current version 5.0
        I don’t use it much anymore because I’ve really fallen in love with Darktable. It’s really similar in power, but has no masking or brush capability – so I would have to rely on GIMP for additional editing. It works well, though, is very stable, and is Windows compatible.
        UFRaw: current version 0.22
        In my opinion UFRaw is the simplest to use of the three and handles noise reduction nicely. If you don’t want to spend a lot of time editing and like an easy interface while still shooting in RAW, this is probably for you. It also exports to GIMP for further editing.
      Stitching and stacking

        Hugin: current version Hugin-2016.2.0
        Simply phenomenal software for stitching multiple photos for creating panoramas or stacking photos, e.g. for multiple exposures. Gives a ton of control over exposure, control points, you can use masks, correct for lens distortion and even create lens profiles. Just get it.
      Command line tools

        Image Magick: current version 7.0.5-3
        An indispensable command line tool. Aren’t familiar with the command line? This is what you should use to become familiar with it. I’ve used it extensively for scripting batch resizing for web uploads – it’s much faster than resizing and exporting through any other program. I have another script using it that blends two images to create an averaged image that helps create really smooth time-lapses.
        Exiftool: current version 10.47
        Every once in a while I need to read and write metadata for a photo. GIMP strips metadata from tiff images when imported and I use this to put it back in. I also use it to time-stamp images (usually just for some time-lapses). It’s useful and worth having even if you only use a few times.
      Image organization

        F-spot: current version 0.8.2
        I still use F-spot for importing photos from my camera/card, but don’t rely on it for tagging anymore. I don’t know how long I’ll keep using it – it frequently crashes on my new system. But, it’s there and it’s been nice in the past.
        Shotwell: current version 0.15
        It’s the default photo manager for Ubuntu. I’ve never used it for organization, but seemed to view tiff images better than a lot of built-in software.
      HDR

        Luminance: current version 2.3.1
        I don’t really do much with HDR. It’s a tool that is good to learn how to use and, perhaps better, learn when to apply it. For a few instances, I’ve been happy to use Luminance HDR. Sidenote: in my opinion, creating HDR images from bracketing is outdated when you consider the amount of information contained in a single RAW file that’s properly exposed. HDR processing is extensively (over)used to create dramatic looking images that in my opinion often lack other aspects of a good photo like composition and prevents young photographers from learning about light. Use responsibly.