G'MIC (GREYC's Magic Image Converter) is a tool that comes with a lot (more then 190) of pre-defined image filters and effects for GIMP and is available for Window, Linux and Mac OSX.

Raw Therapee is a free, multi-platform alternative to Adobe Photoshop Lightroom (RAW converter and digital photo processing software).

PhotoFilmStrip is an open-source application (works on both Linux and Windows) which you can use to creates movies out of your pictures in just 3 steps. First select your photos, customize the motion path and render the video. There are several output possibilities for VCD, SVCD, DVD up to FULL-HD.