Here's a nifty "sim" many of you probably aren't familiar with:
Otherwise known as Outerra. Basically, a guy or team or whomever, has been working for years on creating the perfect procedurally generated world engine. I've been following the project for five or six years, and he's come a long way. Simply amazing is the best way I describe what I've seen. I think the registered version has some helos and planes, but I haven't tried it in a year, so I'm not sure what comes with the demo. I recall last time I flew around in a plane using my Track IR which was pretty neat.
What's really cool is you can start on the ground and fly all the way up into space, and back, and the view is breathtaking at both altitude and on foot, as the procedural engine generates all the details you'd expect from both extremes, including nice clouds, terrain that casts shadows, and little pebbles and stones on the ground with grass.