Are you looking for multiplayer for FS or CFS?
For CFS:

The only place I have heard of is the zone(I don't have CFS)


For FS:

Are you looking to just play around with other pilots or,are you looking to do some serious ATC flying?
If you just want to play around,I hear the zone is pretty good(I've never tried it though)
If you want to do some serious flying under ATC control,the use either VATSIM ( http://www.vatsim.net/ )
or IVAO ( http://www.ivao.org/ ).If you want to do this,I recommend joining both and seeing which one you prefer.

Hope that helps.

www.vatsim.net

I don't need to say anything else.