This past Saturday, I gave a talk at the Toronto SharePoint Camp on building composite applications. I started talking in general requirements terms of why composite applications are useful, what they are, and what are the platform requirements - it naturally came down to a SharePoint demo - it's a great platform for building web based composite applications. The nice thing about SharePoint is that much of that work can be done in an ad hoc fashion. This means less plumbing code for us developers and we get to focus on solving business problems.
Some of the things I demonstrated are all available with Windows SharePoint Services (free). For example, Document Libraries and Custom lists, along with the excellent Outlook integration (including offline support) not to mention version control. Then we got into Workflow and integration with enterprise data with the Business Data Catalog of which requires the Microsoft Office SharePoint Server or MOSS 2007 which is not free (approx $5000), but a totally worthwhile investment. You can easily save the license fees several times over in reduced development effort. I also used SharePoint Designer which is about $200-300.
My slides and demo files are attached. Let me know if you have any questions. I've also included some demo script notes in the slide notes for those who asked.
Also, check out Rob Windsor's Pictures on Flickr of the event