Nope... at a minimal level you just need the sim and an internet connection and a place to fly online.

You can use some "add ons" if you like.... but not "necessary".  One of the best is Teamspeak... which allows you to communicate via voice in real time while flying.  Another payware one that is in widespread use is FSNavigator .... which I don't have.

You do have to set up the sim and your main computer correctly to get lonto the varuious online flight servers though.  See the threads here about getting online and about openiong ports and firewalls and such.

When you get it working... it COMPLETELY changes the experience to fly around real people and with real ATC.

best,

................john