Last year Fritzing came along as a virtual Arduino sandbox, a way to simulate a breadboard and components and run Arduino code to test the circuit. Now VirtualBreadboard, which has been around for a decade, has expanded to simulate Arduino, too. I haven't tried it out, but it looks very handy for travel coding (which I've been doing too much of lately).
BTW, I hear that Fritzing lost their German government grant, so I'm not sure what effect that will have on the project's development. It is an awesome concept, improved by the ability to order a PCB of your design, so I hope they can find other funding sources soon.