123D Circuit is a handy, web-based tool that allows you to create, share and collaborate on circuit designs such as those you'd use with an Arduino microcontroller. And if you’re using an Arduino in your circuit, you can even code inside the embedded IDE and simulate its behaviour directly inside your browser – with almost no risk of blowing it up!
You can also embed these circuits into web-pages, which is clearly sweet music to bloggers' ears. ;-)
For instance, here's a quick circuit I threw together based on the first Arduino project I talked about on this blog, albeit with minor adjustments to the resistance applied to the LEDs (basically to make the results more visible during simulation):
Click on “Simulation View” above to see how the circuit looks when running:
Pretty cool stuff (not the circuit itself – the ability to create it online and embed it in an HTML page :-). Be sure to head on over and have a play.