I've used Wikipedia for some years, of course but only in the past year or two have I experimented with other uses for wiki software. One of my courses is making use of the Encore system and I set up a wiki for my podcast on Friday using PBwiki; according to the website "250,000 educators" used the system (I wonder if users are defined broadly to include all educational users or if it means just teachers and professors?). I'm finding wiki software to be a useful way to manage colloboration so far and did not find it at all difficult to get started. That said, I find it works best as a supplement to in-person interactions and meetings; it might be more difficult to sustain participation if the project was online only.
On another web 2.0 note, I started using Second Life a few days ago since the SLA was holding a student event there. I find the interface a bit challenging to use compared to similar environments I've used MMORPGs are mainly what I'm comparing Second Life to. Compared to podcasting, blogging, social cataloguing/tagging, the immersive environment presented by Second Life has required the most time to learn. At this stage, I'm not seeing much there to bring me back as a regular user. That said, I am planning to visit it periodically to see what I can make of it.