Paint.net is a free powerful image editor for Microsoft Windows devices that, as the name suggests, requires the Microsoft .NET Framework.
I've been searching for a Photoshop replacement for a while, and the obvious choice was GIMP, although I never really liked it much. I am 100% sure that GIMP is an awesome application, but to me it just doesn't feel right.
Gimp always felt cumbersome, especially when it came to the program startup, or the loading of image files.
So instead, I gave Paint.net another go, an application which I have tried before, but gave up because I heard GIMP was so awesome.
Paint.net is a graphical application just like Photoshop, but without many of its fancy tools. If you just need some cropping, resizing and positioning action for images in your blog, Paint.net will be great. At around 6Mb in size, it doesn't take a toll on your system either.
For me it works quite well for basic web design too, simple gradients, some web 2.0 style reflections, nothing too complex. The two things which I miss most about Photoshop though is the text capabilities and vector graphics. Text capabilities in both GIMP and Paint.net are shabby compared to PS, and Paint.net doesn't really have great pen tools either. If you need way more power than the simple Windows Paint, but way less than Photoshop, Paint.net is a solid, well built, lightweight image editor for you.
Update: I have been using Paint.net for quite some time now, mostly for basic image editing operations like resizing, cropping or manipulating images. It is fast loading which is always a plus and lightweight on the system. After having tried many, some of which were either to heavy in functionality or resource usage, Photoshop and GIMP come to mind, I kept using Paint.net as it is the ideal program for everything that I need to do image manipulation-wise.
Paint.net offers a lot of features on top of what I need that may be useful to designers. This includes layers, special effects, tools for a variety of tasks and more.