Matt Webb's overview of Social Software is pretty useful - particularly as it is a n=hub of useful links out to specifics - for instance, this fascinating article on Life With Alacrity (and Comments!) on optimal group size and "The Dunbar Number", which I for one had never heard of.
UPDATE: Via Many to Many, I came across the Online Community Toolkit, which looks like an excellent set of resources.
UPDATE 2: an excellent matrix of social software facets (not orthogonal!) - could help in deciding on a particular tool, or in gap analysis to work out how to make a 3600 networking tool.