OpenSourcePhysics is a set of classes for writing educational Java applets and applications. I'm learning the library and am working on some OSP projects.
To run OSP applications, you must have Java installed. You can install the SDK (which comes with the programs you need to compile java code as well as run it) or you can install the JRE (Java Runtime Environment) which allows you to run Java applications and applets. To download an installer for Java SDK or JRE, go to http://java.sun.com/.
Projects
Canonical Spaceship
This app allows students to explore the relationship between force and motion by allowing them to control four thrusters (in the +x, -x, +y, and -y directions respectively) using the keyboard.
