Wikis in Action — The Top 57 Wikis by Rank
This is a list of the top 57 wikis (by rank). It's interesting to see how they are being used and which wikis they're using. One thing Katriel Reichman said in the wiki interview the other day is that wikis are often best used when the topic is reference information. I haven't gone through each of the 57 wikis, but I bet one could learn a lot by perusing them for a few days.
I found this list by searching for wikis in del.icio.us, the most popular social bookmarking site.