Blender - 3D Software
Blender is the free open source 3D content creation suite, available for all major operating systems under the GNU General Public License.
The website contains examples of some of the amazing content which has been created with blender, including this movie:
GeoGebra - Free Maths Software

GeoGebra is dynamic mathematics software for all levels of education that joins arithmetic, geometry, algebra and calculus. On the one hand, GeoGebra is an interactive geometry system. You can do constructions with points, vectors, segments, lines, conic sections as well as functions and change them dynamically afterwards. On the other hand, equations and coordinates can be entered directly. Thus, GeoGebra has the ability to deal with variables for numbers, vectors and points, finds derivatives and integrals of functions and offers commands like Root or Extremum. These two views are characteristic of GeoGebra: an expression in the algebra view corresponds to an object in the graphics view and vice versa.
GIMP - Free Image Manipulation Software
GIMP is an acronym for GNU Image Manipulation Program. It is a freely distributed program for such tasks as photo retouching, image composition and image authoring.
It has many capabilities. It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, an image format converter, etc.
Google Earth
Google Earth lets you fly anywhere on Earth to view satellite imagery, maps, terrain, 3D buildings, from galaxies in outer space to the canyons of the ocean. You can explore rich geographical content, save your toured places, and share with others.
Google SketchUp
Create 3D models and share them with the world
Google SketchUp is software that you can use to create 3D models of anything you like. Build models and add them to Google Earth »
iTunes U - Free audio and video resources
iTunes U is a part of the iTunes Store, featuring free lectures, language lessons, audiobooks and more that you can enjoy on your iPod, iPhone, Mac or PC. Explore over 50,000 educational audio and video files from top universities, museums and public media organisations from around the world. With iTunes U, there's no end to what or where you can learn.
Apple's free iTunes software (available for Mac and PC) is required. Participating universities include prestigious British institution such as Oxford, Cambridge, Warwick and the Open University.
Open Office
Compatible with other major office suites, OpenOffice.org is free to download, use, and distribute. Download it now, and get:
Scratch - Animation
About Scratch
Scratch is a new programming language that makes it easy to create your own interactive stories, animations, games, music, and art -- and share your creations on the web.
Scratch is designed to help young people (ages 8 and up) develop 21st century learning skills. As they create and share Scratch projects, young people learn important mathematical and computational ideas, while also learning to think creatively, reason systematically, and work collaboratively.
Stellarium - View the Stars

Stellarium is a free open source planetarium for your computer. It shows a realistic sky in 3D, just like what you see with the naked eye, binoculars or a telescope.
It is being used in planetarium projectors. Just set your coordinates and go.