Pinta is an open-source, multi-platform attempt to recreate the Paint.NET experience. It's described as "early in development," but for a 0.1 release, feels majorly on its way. It supports multi-layer editing, runs on Linux, Mac, and Windows (with some additional support installed), and a lot of the interface is already in place. I tried it out to edit, crop, and tone some images for this morning's Lifehacker posts, and I could see working it into my rotation, as it's a bit lighter and easier to get around than the GIMP, the other cross-platform image editor of note. Read more: Lifehacker
Official site: Pinta