Sumatra, alongside maybe Everything and IrfanView, is my favorite piece of software on windows. Incredible speed, great customizability and awesome compatibility. I really missed it when I made the switch to Linux, but not enough to consider installing WINE. Still, absolutely wonderful project deserving of the highest praise.
Paint.NET is great, too. It's not open source, unfortunately (for valid if regrettable reasons), but it's free as in free beer. The memory of its GUI makes me sigh a little whenever I open GIMP.
Well now you got me curious. Why is that?