This page is constantly updated and still needs a lot of work. I'm just trying to put together a list of all image tools, both online and downloadable.
Online photo-editing tools
Sometimes you don't have to download anything to edit photos. Do you want to rotate a photo, resize, blur or sharpen, change saturation or brightness, remove red-eye? All of this, plus more can be done in your browser.
Photopea.com - recommended online photo editor. Choose this one if you are a little bit skilled / used to use Photoshop in the past. Can even open PSD files.
Polarr - great if you need to quickly edit photos online, lots of adjustments tools, and filters. Easy to use face retouching. Full editing history. Also possible to download as an application for Windows and Mac.
BeFunky.com - easy to use photo editing and retouching tool, also useful as collage maker
Photorec 7.1 - opensource and multi-platform, doesn't have a fancy graphical user interface, but it's well documented so don't be scared
Exif Untrasher - photo recovery tool for Mac. Supports a wide range of storage mediums (SD card, CF card, memory stick, hard disk ...)
Recuva - the free version is enough (Windows only), but only very basic tool, I have tried it several times and quite often I had to use somehting more powerfull