Recs.
Updated
Paint.NET is free image and photo editing software for PCs that run Windows. It features an intuitive and innovative user interface with support for layers, unlimited undo, special effects, and a wide variety of useful and powerful tools. An active and growing online community provides friendly help, tutorials, and plugins.
It started development as an undergraduate college senior design project mentored by Microsoft, and is currently being maintained by some of the alumni that originally worked on it. Originally intended as a free replacement for the Microsoft Paint software that comes with Windows, it has grown into a powerful yet simple image and photo editor tool. It has been compared to other digital photo editing software packages such as Adobe® Photoshop®, Corel® Paint Shop Pro®, Microsoft Photo Editor, and The GIMP.
SpecsUpdate
Pros
Pro Easy to learn and use
Unlike most photo editing software, Paint.NET is simple to learn and easy to use. This leads to less time learning the in's and out's of Paint.NET and more photo editing. For those who only edit photos sporadically and don't have time to invest in learning complicated tools for editing, Paint.Net will suit your needs.
Cons
Con Not really comparable/ in same league
Not really comparable/ in same league as other entries on this list, which are full featured, advanced image manipulation software (e.g. Photoshop, Gimp, Capture, Light/ Darktable). Would be similar to comparing Notepad with Word. Must be why MicroSoft gives it away.
Con No project-based interface
You can only edit a single image at a time in Paint.NET. For example, in other tools you can open 5 or a 100 images at the same time, and stack those windows in all directions.
Furthermore, when you are editing in Paint.NET you can only "view" a single image at a time, even though you can have multiple images open. This limits your zooming ability, as the image will go fullscreen quite quickly, and hide parts of it behind the floating toolbars which can not be embedded anywhere on the screen.