Some very nice multiuser Unity applications have been released over the past couple of months. Here's a list:
atari's games.com
Atari uses Unity to power their well-known online gaming site, games.com. Games implemented using unity include: Battleship, Boggle, Monopoly, Scrabble, Sorry!, and Yahtzee.
Vyew
Vyew is a free webconferencing tool that includes screen sharing, file sharing, whiteboarding, chat, shared searching.
Seite1girl
Seite1girl is a live exhibition promoting a line of lingerie for the German clothing store C&A. Every evening from 20:30 to 24:00 (German time), a video projection in front of the store displays models wearing the lingerie. The specific poses are chosen by users online at http://seite1girl.de, and the reactions to those poses by passersby are broadcast via streaming video back to the web site. The project is a collaborative effort by the newspaper "Bild Zeitung" and C&A.
Tampopo
Tampopo is an art exhibition by Kentaro Yamada in which users can blow the seeds off a virtual dandelion. One blow can affect not only the dandelion in the gallery but also other dandelions on the network (or internet). A number of viewers can blow each other's dandelions from remote locations. For more information on Yamada-san's work, see: http://www.kentaroyamada.com