You can now install the FlightGear Flight Simulator and use it (in view-only mode) to display a 3D simulation of the vehicle and its surroundings.
This provides a much better visualization than 2D maps/HUD alone (as provided by MAVProxy and Mission Planner). In particular it is much easier to see movement that is hard to judge properly in 2D - like pitch and yaw.
SITL outputs FlightGear compatible state information on UDP port 5503. All you need to do is install FlightGear and start it using the batch files provided. When SITL starts, the FlightGear window will sync to your current location.
Currently this feature is only available on the master ardupilot branch. Setup instructions are available here: Windows and Linux (Ubuntu 14.04).
Kudos to Tridge (yet again!) for making this happen!