I just see the demo of TopBraid Composer. It is a terrific
database-based, collaborative, highly scalable, much more faster (than
protege) ontology editor.
http://www.topbraidcomposer.com/

You can have a free download (1 month use) from the above url.

Heard from Holger Knublauch (http://www.knublauch.com/, who is also
the developer of Protege-OWL): protege multi-user mode actually does
not work, because no any control of conflicts - all changes are at the
triple level and are propagated immediately to all users.

TopBraid Composer has sort of modular ontology support. It allows an
ontology to be decomposed into several files, and lock some of them if
necessary.

It is highly interesting if we can adapt it for our usage (e.g. to
improve COB -Editor). It is not open source but has a plugin.
Educational license is only 400$.

-- 
Jie Bao