Definition of Third-Party:
Third-Party has nothing to do with multiplayer. It means anything that is made by someone that is not officially recognized by the original developers of a product. First-party would, of course, be the developers themselves. Second-party would be those not connected with the development team whom the developers ask specifically to make support software. Third-party, as I said earlier, are independent "parties" (those would be people in this instance) who make support software of their own volition.
I hope I cleared this up for you.

As posted before, check out Smiff's support site for information on plugins that both come with the emulator and those made by others (NRage and Azimer, again, are the ones I use).