I use Paint.NET for a lot of things, most of them professionally. If I someday get lucky in this startup lottery thing, I intend to donate some thousands of dollars.
EDIT for add: I see a lot of comments talking about using Paint.NET for croping and resizing. I do so much more than this. I actually create all kinds of communication materials with it. Flyers, images for a landing page, business cards, etc. The layers feature is what make it of great value for me. Also the feature to open different images at the same time is great to create an image that includes a logo, a stock photo, a standard background.
Two things that are missing: be able to save it using CMYK color scheme (better for professional printing) and saving as PDF (easier to send it to graphic stores).