library.io - Learn electronics and easily create circuit boards online
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.
Circuito.io - Instant code and wiring for your project
Algodoo - Algodoo is a 2D simulator freeware product designed as a physics learning tool. It was originally created by Emil Emerfeldt as part of his master’s thesis in 2008. Read more about Algodoo.
Circuit Stickers - Peel-and-stick electronics for crafting circuits.
LabsLand - LabsLand lets students access real educational laboratories through the Internet.