Agave is a very simple application for the GNOME desktop that allows you to generate a variety of colorschemes from a single starting color. (GPL)
Linux - FreeBSD
Hugin is a cross-platform open source panorama photo stitching program. (GPL)
Linux - FreeBSD - Windows - MAC OS X
Luminance HDR is an open source graphical user interface application that aims to provide a workflow for HDR imaging. (GPL)
Linux - MAC OS X - Windows
Mockup Design is a mockup tool that uses the local storage of a browser as a backend. You can integrate with any RESTful API or any Backend service. (MIT)
Cross-platform
The Pencil Project's unique mission is to build a free and opensource tool for making diagrams and GUI prototyping that everyone can use. (GPL)
Cross-platform
6 applications