I've been checking out the Microsoft Office Sharepoint Server (MOSS) 2007 and it seems to be quite cool for document storage needs and for creating a Wiki.
It can also be used by power users to create simple applications for meeting short, temporary requirements such as keeping track of volunteer assignments, recording contributions by donors etc. MOSS can be extended with .NET and several applications have been developed over MOSS to reap the benefits of the search engine, content management and security. You can check out http://www.glu.com/emea to see a completely skinned MOSS site.
Microsoft also offers the Fantastic 40 site templates, which are like pre-built applications using Sharepoint.