The WSS-only deployment (as promoted extensively by Mike Walsh and others) has always been somewhat underused, partially at least because people didn't really understand the features and facilities which were available in WSS as opposed to the full-blown Sharepoint Portal Server product.

Now in the 2007 world with WSS v3 and MOSS 2007 with both products having an enriched function set, Microsoft have released an easy-to-scan guide for checking off your favourite bits of functionality and seing which of the various SKUs it appears in.

http://office.microsoft.com/en-us/sharepointtechnology/FX101758691033.aspx