The PhET Interactive Simulations project at the University of Colorado Boulder creates free interactive math and science simulations. PhET sims are based on extensive education research and engage students through an intuitive, game-like environment where students learn through exploration and discovery.
edX is a non-profit, massive open online course (MOOC) provider. The majority of edX courses are entirely free to access and most also offer an optional verified certificate track for a fee that varies per course. Courses may consist of video and text content, discussion forums, and a number of problem and assessment types.
Coursera is an education company that partners with the top universities and organizations in the world to offer courses online for anyone to take, for free.
NUS has signed up to Coursera to offer its own MOOCs.
A collection of virtual labs, scenario-based learning activities, and concepts tests which can be incorporated into a variety of teaching approaches as pre-labs, alternatives to textbook homework, and in-class activities for individuals or teams.