Grant Bowman has a huge list of collaborative tools, hosted on sourceforge. It is specifically focussed on open source projects, though it has a smaller number of commercial apps and related links. It's not categorized, and has the usual problems of such lists -- it includes the generic graph-drawing package GraphViz and esr's Fetchmail, for example, so its hard to see a crisp line drawn around collaboration, and the projects are listed alphabetically, so there isn't a sense of functional category. As an object of contemplation, however, or if you are looking for inspiration, it's great.