Non-Linux FOSS: Seashore
As Linux users, we tend to take programs like GIMP for granted. Thankfully, as of version 2.8.2, GIMP is available as a native application for OS X! Because everyone reading this most likely is familiar with how awesome GIMP is for photo editing, it's worth mentioning there is another open-source photo-editing application for OS X named Seashore.
Seashore is an application written in the native OS X Cocoa language, so it clearly looks like a native application when running (Figure 1). Although OS X users no longer have to install X11 in order to take advantage of GIMP, Seashore is a powerful image editor in its own right, and because it's open source, it's well worth the effort to try it out! You can download Seashore at http://seashore.sourceforge.net. The native version of GIMP is available at http://www.gimp.org/downloads.
Figure 1. Based on the interface alone, it's obvious Seashore is a native OS X application.