I work in a research and implementation team for Quest Atlantis a virtual world designed and developed at Indiana University as the brain child of Sasha Barab. It is a series of worlds designed for kids 8 to 14 years of age where they complete games, missions and quests organized around curriculum work and values-based activities. teacher act as curriculum designers and have a whole interface for planning, overseeing and reviewing student work.
It is built on the ActiveWorlds platform but has so much more than visual 3D design added to it by the team at IU. It is fully moderated and we do not permit VOIP for other than teacher in teacher-nly worlds as we have no way as yet to moderate the dialogue. Otherwise the whole site is moderated both by a profanity filter and a human being who scans logs, bulletin boards etc for duty of care issues like bullying, racial epithets, or sexual innuendo. This moderation has proven very important as we get QA into schools where SL will never go. People know this is a protected and safe environment.
This program has been running for over 5 years and is in some 8-10 countries yet is known by only a few. It strikes me as funny that there are many virtual world environments (take a look here Virtual Worlds Wiki) but it really took SL to get us all talking. While I think the SL developers are shameless self-promoters and very opportunistic they have pushed the agenda for all of us.