For those who use and develop using the ArduPilot autopilot software (as featured on DiyDrones and ArduPilot Discourse), a powerful simulation tool is available that can replicate all of the features of the autopilot without flying an actual aircraft. This is extremely useful for testing complex missions or parameter configurations without risking real hardware.
Unfortunately setting it up is a bit tricky, official instructions are available however to make life easier I have created an automatic script that you can run that will install and run the SITL software on a Ubuntu 14.04 Linux PC or virtual machine.
This software is licensed under GNU General Public License version 3 and the source code is available at https://bitbucket.org/dellarb/ardupilot-sitl