This plugin for Paint.NET allows you to "subtract" white from an image, which is helpful for many situations, such as recoloring a background, extracting a logo, or working with scanned drawings or documents.
» Download Remove White 1.0 for Paint.NET
» Download Remove White 1.0 Source Code (Developers)
If you're looking for a similar effect in Photoshop, check out the Eliminate White plugin for Photoshop on Windows and the Peel Off White plugin for Photoshop on Mac.