I just noticed the release of Hydra 1.0. Hydra is a developer focused text editor. What makes it interesting is that it uses Rendezvous (ZeroConf) to facilitate collaboration on a local network. So, for example, two or more developers can effectively do "pair programming" on the same code from different machines - useful for big laptop development sessions, for example.
This isn't the first product of its kind for Mac OS X. iStorm is another Rendezvous based collaborative document environment. While Hydra is focused more on the developer, iStorm seems targeted more on science/math users, with TeX support and interactive blackboards to go along with the styled text. iStorm also houses its own Chat client (I didn't notice if Hydra sports this feature), allowing conversations to happen outside of the document itself.
This is a pretty interesting application of the Rendezvous technology. While generally limited to local networks, it does allow a small group of people to collaborate very easily in a local environment. Imagine being able to bring in people from outside the company to brainstorm for two days and all being able to work on the same document without having to configure large collaboration servers to allow two important but temporary outside users into the system. Or, with Airport, one could even go to a coffee shop or bar and set up a small computer-to-computer network and collaborate - again without the need for a complex client/server set up. And unlike Wikis or anything else web based, all updates can be monitored in real time.