What do you use your wiki for?

In my day job I'm a librarian. A big part of proving my value to the bigger organization is showing how often the library is used. I use my wiki to track reference statistics like user type, length of question, and what type of things they're asking. Archiving these statistics month by month has given me a great database of monthly trends and FAQs. I also use my wiki to store the plethora of passwords generated for each online database subscription I have. My wiki is a lifesaver!

I've used it to plan a get-together for co-workers from a former job. I had everything set up in evite for the event, had gone through the whole process of choosing templates, cute "Yes, I'm coming" and "No, I'm not" replies, etc etc and then evite crashed my browser at the very end before I had saved and lost all my work. I was so annoyed that I just decided to take a less cute approach and send everyone a bare-bones wiki and ask them to just write their names under their choice of the 2 dates I suggested, or under "Sorry, I can't go". It worked out just fine, and introduced the former co-workers to wikis!