Paint.NET is essentially a dumbed down Photoshop that can be downloaded for free.
...What, you wanted more? Oh, alright, I guess I should put actual effort into this.
The program has many of the simpler capabilities for modifying and creating images, such as cropping, creating new shapes, photo effects such as grayscale and blurring, a layers system, gradients and alpha transparency...believe me when I say that this really is just a simplified version of Photoshop, or if you want to think otherwise, a greatly upgraded version of Windows' default Paint program. This is by no means a bad thing.
The interface, while it overall requires a .NET 3.5 download, is quick, intuitive, and simplified. An info bar at the bottom of the program tells you what tools can do based on what you do next with said tool, and it can export to various file formats. Still want to do more with Paint.NET? There are plugins for extra features and file format exports on their forums, developed by other Paint.NET users.
I actually own an older copy of Photoshop, and I got rid of it for Paint.NET. Who wants to wait a good minute for everything to load up on Photoshop properly when all I'm working on is a simple screenshot modification most of the time? 5-10 second boot time, I'm seeing a blank canvas and I can get straight to work. Don't get me wrong, if you need to do the professional stuff, pay the money and pick up Photoshop (or use GIMP, another $0 product). Otherwise, save the cash for something really worth purchasing, like a Wii, and grab Paint.NET instead. Also, 5 points if you can name the thing (if you can call it that) I drew.