Terraria - Dig, Fight, Build! The very world is at your fingertips as you fight for survival, fortune, and glory.
PhET Interactive Simulations - Founded in 2002 by Nobel Laureate Carl Wieman, the PhET Interactive Simulations project at the University of Colorado Boulder creates free interactive math and science simulations.
SimPhy - Interactive 2D & 3D Physics simulation software
LabsLand - LabsLand lets students access real educational laboratories through the Internet.
Physion - Physics Simulation Sandbox
BookWidgets - BookWidgets is an excellent application that let you create engaging exercises in minutes and easily share a varied variety of beautiful and fun activities with your classroom and others.