I'm a regular user of wikipedia, but I was really surprised by the breadth and amount of wikis that have been created. It's such a useful communication tool especially for professions such as my own (conservation) which are small in number but rely so much on cross communication and collaboration with professional colleagues. It's always nice to have a little community.

Here's an awesome wiki created by the book conservators of New York City

I especially like the food sign up table for events. Conservators are pretty much obsessed with food.