I'd endorse everything that Rod suggested, but as well as SocialText as an Enterprise class wiki, you might also consider JotSpot, or Confluence from Atlassian.
You should also take a look at the whole family of Web 2.0 office tools that are being discussed at next week's Office 2.0 Conference that Rod and I are attending next week in San Francisco. Take a look at ITRedux's Office 2.0 database for a complete range of the available applications and web services.
