Science magazine has instituted the Science Prize for Online Resources in Education [SPORE], to recognize worthwhile efforts in science education. Ars Technica reports that the current prize was awarded to the Open Source Physics [OSP] project, launched by a group of college professors. The site offers simulation software for the physical sciences, as well as other educational materials, such as lesson plans.
The OSP Collection provides curriculum resources that engage students in physics, computation, and computer modeling. Computational physics and computer modeling provide students with new ways to understand, describe, explain, and predict physical phenomena.
The software is written in Java, so it is useable on multiple platforms (e.g., Linux, Windows, Mac OS X), and is distributed with source code under the GNU General Public License [GPL]; this means that you can modify the software to meet your requirements. You can also distribute the modified version, provided that you do so under the GPL. The OSP site has a wide variety of materials for various areas of physics, as well as a few applications for other sciences.