Windows/Mac/Linux: If imitation is the sincerest form of flattery, the coders of Pinta love the heck out of Paint.Net. Luckily, they also want to make it available on every OS, and are faithful to its just-enough interface methodology.
If you've read Lifehacker for a bit, you know we're admirers of Paint.Net. It's lightweight, free, and does most of what the average home user would want out of a photo editing and painting application, without making them learn an entire realm of commands and advanced photography terminology. It opens Photoshop files, it touches up images nicely, and it doesn't cost hundreds of dollars, so we dig it.
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.
Pinta is a free download for Windows, Mac and Linux systems. Ubuntu and Fedora readers should hit the via link for a quick tip on making installation easier than compiling from source.