Ever wanted a more complete simulation experience? ATC staffed by real people? Traffic that wasn't just preproprogrammed? If you haven't seen it before you should go check out VATSIM at http://vatsim.net/ It's a multiplayer experience for flight simmers and air traffic control simmers. Hundreds of pilots and controllers connect everyday in the virtual world and all the servers are connected to each other.
Flying on VATSIM is an amazing experience once you get past the complicated setup procedures (all of which are documented in their Pilot Resource Center guides). It is not for everyone however, if you do not wish to adhere to strict regulations it might not be for you. This is because the VATSIM experience is modeled as close to real life aviation as possible including all communications and operating procedures. Unlike real life though, if you have a desire to learn most controllers or pilots are more than willing to help in anyway they can.
leadZERO is right on here. It's an incredible experience for both pilots and controllers. One important thing he didn't mention: it's FREE. And all the necessary software is free. But you do need to register with VATSIM to get a user ID and password, so they can deny you access if you can't behave yourself (makes for a wonderfully sane environment). Check it out.