Groove "desktop collaboration software". This is potentially a big player in the field - founded by Ray Ozzie (who created Lotus Notes) and financially supported by Microsoft. The home page claims it "effortlesslly keeps people, information, and PC's in sync; online or offline, real-time or anytime - it just works" and that it is "10x better than email alone for sharing files or working with others on documents, tasks, projects and decisions". It costs about $99 and a free preview edition is available. For a critical review, read Robin Good's (2003) Groove: Ten good reasons not to buy, which complains (among many other things) about the way Groove hogs processing power and bandwidth and its un-intuitive user interface. A positive take on using Groove in education is provided by Rick Lillie of Cal State, as described in this (unfortunately marred by marketing-speak) article on the Groove website and (more neutrally) on Lillie's blog. Lillie uses Groove in combination with web pages, SharePoint, electronic books, printed books and MSN Messenger.
Windows SharePoint Services, is a heavily promoted Microsoft's product providing workers with a common environment in which to share information and documents.
Macromedia Contribute. Potentially another big player. At $75 a shot, "Contribute enables a large group of non-technical participants to collaborate on a shared project. In other words, not only does Contribute excel as a tool for updating publicly available web pages, it also enables non-technical users to share hyperspace, collaborating on documents through a browser-like editor from anywhere in the world."
Socialtext is an online collaboration system incorporating collaborative document editing via wiki blog-like features together with conferencing features.