The TopicMapsLab has launched a new service called Maiana, which is a social topic map browser. The Maiana browser is deliberately made quite similar to the Omnigator so people will find it familiar, but has a couple of new features, like allowing anyone to upload their topic maps, and allowing comments on topics and topic maps.
Interestingly, Maiana is a Ruby on Rails application running in JRuby using JRTM (to be released; more information then), with Ontopia as the backend. So Maiana is actually an Ontopia application.
Ontopia's Blog 