Cinelerra | Open Source Video Editing

"Cinelerra is a professional video editing and compositing software. It is designed for the GNU/Linux operating system. It is produced by Heroine Virtual, and is free software distributed under the GNU General Public License. Cinelerra also includes a video compositing engine, allowing the user to perform advanced compositing operations such as keying and mattes."

http://cinelerra.org/

GimpFX Foundry | free scripts to expand GIMP

"This set contains different scripts for the creation of effects such as the old picture, lomography, image stylization, creation of all possible meshes and frames for pictures. It also contains scripts that significantly extend GIMP options when working with layers.
The set includes a huge number of scripts for work with image color; there are the scripts for total picture editing, creation of inner shadow, picture sphere and many other effects." [1]
(Link)

Geeqie | free image management software

"Geeqie is an image viewer that makes navigation of images easy. Images can be viewed full screen, and a slide show function allows automatic changing of the displayed image.
Geeqie is an image manager, with complete copy, move, rename, and delete functions available at the press of a button. Find the right image with the search feature. Image collectors can free up space with a function that finds duplicate images.
The one thing Geeqie is not, and never will be, is an image editor. However, Geeqie paired with an image editor, such as the Gimp, can be a very powerful combination."

(Link)

Arduino | open source programming and micro-controller

"Arduino is an open-source electronics prototyping platform based on flexible, easy-to-use hardware and software. It's intended for artists, designers, hobbyists and anyone interested in creating interactive objects or environments. Arduino can sense the environment by receiving input from a variety of sensors and can affect its surroundings by controlling lights, motors, and other actuators. The microcontroller on the board is programmed using the Arduino programming language (based on Wiring) and the Arduino development environment (based on Processing). Arduino projects can be stand-alone or they can communicate with software running on a computer (e.g. Flash, Processing, MaxMSP)."

(Link)

Longomatch | open source sports video analysis

"LongoMatch has been designed to be very easy to use, exposing the basic functionalities of video analysis in an intuitive interface. Tagging, playback and edition of stored events can be easily done from the main window, while more specific features can be accessed through menus when needed. Flexible and customizable for all kind of sports Flexible and customizable for all sports LongoMatch can be used for any kind of sports, allowing to create custom templates with an unlimited number of tagging categories. It also supports defining custom subcategories and creating templates for your teams with detailed information of each player which is the perfect combination for a fine-grained performance analysis"
(Link)

Pencil | open source animation and drawing software

Update:
@ibeardslee has pointed me to a forked, updated version of this software called Pencil2D, which appears to be a better option than the link I give below.
(Link)
"Pencil is an animation/drawing software for Mac OS X, Windows, and Linux. It lets you create traditional hand-drawn animation (cartoon) using both bitmap and vector graphics. Pencil is free and open source.

Pencil is not intended to mimick web-oriented vector-based animation software such as Flash. Its main purpose is to make traditional animation. Neither does it try to rival commercial software targetting the professional animation sector. Pencil is intended to be a simple programme enabling anyone to make 2D animation."
(Link)