as far as i know RCU is the most well known site for 3D nitro.....the flying cirkus site is more along the larger model side of 3Ding and Rcgroups is geared more for electric 3Ding

there is also the Probros and they're known for profiles

http://www.flyingcirkus.com/default.asp

http://www.rcgroups.com/forums/index.php

http://the******************.com/