Forgive me if someone already posted this, as I've not been following the thread closely this week:
In my quest too find interesting 3rd party tools/addons that bring more meaning and immersion to the sim I've run across a little gem called "Self Loading Cargo."
Self-Loading Cargo is a real-time simulation of Cabin Crew, Passengers and Ground Crew for flight simulators.
www.selfloadingcargo.com
It's basically a passenger system for flight sims. Currently it's geared mainly for airliners, but you can use it for smaller GA planes, too, provided you download (or make) an appropriate cabin map file. The developer seems very engaged with the community and it's being worked on quite a bit. I've done a couple of flights in the AirBus 320 and it's really added a nice passenger satisfaction component to those flights. You have to manage your flight crew and attend to the passengers' needs (drinks, entertainment, etc.) while not subjecting them to too high or low Gs, high bank turns, etc. If you do you freak our your passengers and your approval rating will drop. (Also, you hear them screaming which is a nice touch).
All in all a nifty little add-on that is well worth the 15 pounds or so asking price.
Supposedly cargo is coming at some point (not the self-loading kind), as well as better and more GA support.