Now you can also simulate and see the result of your Arduino project via Arduino simulator called Virtual Breadboard.Not only arduino you can also simulate various hobbyists microcontrollers like PIC16 and PIC18.
It is available in two versions VBBExpress and VBB,in which VBBExpress is available for free download but you can not simulate,it is just for prototype and to simulate you need to get VBB for which you should pay to get it.
There are some good resources available to learn about this software.
|Cost||Free||Pay for Download|
|Linux/Mac||Yes, with Mono runtime||No, not yet fully Mono compatible|