FrenchToast - there was in fact another site a long time ago that named itself "Renderocity" by a long time Renderosity member, well known & skillful merchant. At no time do I recall her site seemingly to fall under attack or given the cold shoulder by RO because of the knock-off name. And to me, that one is a blatant knock-off. There is also currently a site called Renderotica - which is self explanatory in name, but that I would not consider a knock-off at all of Renderosity.

Also, to name a few others:
3D Commune
3D Arena
Poser City
Poserworld
Poserstyle
Rendervisions

Part of the reason for 'similar' sounding names is evident to most - it involves 3d/Poser and they want the name to stand out as such - and that will end up with some names sounding very much the same. And of course, you will have the people who want to bank on knock-offs of a site's name - I'm in NO way saying any in that list are such, btw.

The reason RO has their webring is to link sites of a similar nature - this by their own admitting. And many of them are sites that either operate independently as a community itself and/or have 3d items to purchase. RO has been growing convulsingly and in similar fashion changing how they operate as a business and allow access to certain 'proprietary' (as they feel) information and/or areas. This is their right as a business - but as a business that runs a 'community', they are going to upset, dissapoint, and down right piss off a lot of people who are sure gonna let them know it.

RO does not discourage cross-site knowledge in terms of trying to stifle direct competition. They consider themselves to be number 1, but certainly not the only player in the game - and hey! - they'll even show ya where the others are, just jump into their ring. :)