Pilot passed out in a small GA plane. What can a passenger do?
Best case scenario: You're straight and level, on frequency with some form of human being, there's no immediate danger and you have the know-how to transmit. In that case, that human will provide you with everything they possibly can to help you. Most important thing for you to do is keep the aircraft away from clouds, away from terrain and not panic. You'll probably be in an aircraft that is more-or-less easy to fly in this situation. Most GA aircraft are quite stable and will not be a beast to control.

Once talking to ATC they will tell you to set your transponder to a number. You should find somewhere a 4-digit readout with a knob below/above each digit. Set the number indicated to help ATC identify you easily.

Otherwise, look around you for big landmarks (Mountains, Lakes, even a large Church/Cathedral) as one of the first things ATC will want to do is work out where you are.



That's best case; let's look at some alternatives. 

You're out of range of the tuned frequency on the radio, or there is no response: 

Tune 121.5MHz, this is the emergency frequency. Someone will be monitoring this channel and be able to give you further advice - if nothing else a local ATC channel to tune. Failing this see if your pilot has a chart handy, he may have drawn his intended route on it - look along that route for anything that looks like a frequency to tune into the radio.

Aircraft is climbing, or descending:

Get the aircraft into safe, level flight. Remember controls work in the opposite sense to what you might expect (probably not if you've ever played any flight simulator). If the engine is roaring reduce power slightly. If you appear to be losing altitude with the nose on the horizon increase power slightly. Do everything with tiny incremental changes. Try not to fiddle too much with controls, only what is necessary.

Aircraft is heading directly for a cloud/out to the sea/towards a heavily built up area/a big airport:

Turn 180 degrees - don't try to turn too quickly, it should take roughly 30 seconds to 1 minute to complete this turn.



There are courses which are aimed at non-pilots who regularly fly in the right hand seat. Here's some interesting reading:

http://www.avweb.com/news/safety/183023-1.html