Open Source Software

Cellumat3D
Open source tool for simulating and exploring cellular automata in 3D space
http://cellumat3d.sourceforge.net/

3D-Desktop
GNOME Linux OpenGL program for switching virtual desktops in a seamless 3-dimensional manner.
http://desk3d.sourceforge.net/

DSVideoLib - DirectShow Video Processing Library
Free software library for device-independent access video images under Microsoft Windows.
http://sourceforge.net/projects/dsvideolib

DXF to OpenGL
An OpenGL program, with source code included, that displays DXF files.
http://www.bearcave.com/dxf/dxfintro.htm

Image Restoration and inpainting
MinGW, OpenGL, Visual C++ and Java image processing project about enhancing, denoising, restoring and detecting/removing parts of images/pictures.
http://restoreinpaint.sourceforge.net/

Jahshaka - Realtime Effects
Video and film composting and special FX system that uses OpenGL hardware rendering to give operators near real time interactivity. The system is cross platform, and includes full CG, paint and image processing modules.
http://jahshaka.org/

The Open Toolkit
An OpenGL library for Mono and .Net. Contains information, documentation, and discussion forums regarding the open-source OpenTK library, as well as C# and OpenGL in general.
http://www.opentk.com/

OpenUniverse
Free space simulator. Currently focusses on the Solar System and lets you visit all of its planets, major moons and a vast collection of smaller bodies in realtime 3D with OpenGL.
http://openuniverse.sourceforge.net/

PPE
The Pretty Poly Editor (PPE), is a free and open source 3D Modeller. (GNU GPL)
http://prettypoly.sourceforge.net/

Really Slick Screensavers GLX Port
Port of OpenGL screensavers that were originally written for Windows to GLX. It is intended for use with an existing screensaver daemon like xscreensaver.
http://rss-glx.sourceforge.net/

Stellarium
Shows a realistic sky in 3D, just like with the naked eye, binoculars or a telescope.
http://www.stellarium.org/

Terraform
Free and open source interactive height field (or fractal) terrain generation and manipulation program. OpenGL is used for rendering and for displaying the height field (or fractal) terrain. (GNU GPL)
http://terraform.sourceforge.net/