The MSDN Library has been offering Wiki like functionality for quite a while now (though I'm yet to see it really used). What's interesting is that this week the library will be open for tagging.

This opens up all sorts of potential in terms of categorising information in a way that is much more useful for people, beyond just the TOC type of organisation that is currently provided by Microsoft.

I'm really curious to see how this pans out, and I'm sure I'll be having a good look at it as soon as it becomes available. Hopefully this sort of thing will eventually make it's way into Sandcastle as well - I could imagine the use of tags in the XML comments flowing through to the rendered documentation as something that could be really useful.