Some of the examples below were found through exploring the web, some were found in Will Richardson's book (see Books (including e-books))
A collaboration of sites and sounds: using wikis to catalog protest songs - This is a very clear example of how a wiki can be used for learning and assessment. It includes a full lesson plan.
Darren Kuropatwa's AP Calculus class (school in Winnipeg)
Book study project (from a Georgia school)
Mobile Machines and Vision Laboratory (MMVL)
Hunter-gatherer wiki (The Ohio State University)
Great controversies in neurobiology (Brown University, USA)
Web-based information resources (Ross School of Business, University of Michigan)
Anne Davis's Write Weblog (school in Georgia, USA)
Nouakchott Writing Classes for students at NKC university
The Blended Learning Unit at the UK's University of Hertfordshire has produced a series of Business School case studies of Wiki use
The following video is a 55 minute lecture, in which Richard Buckland describes how he incorporates wikis into his own teaching. I found this to be both highly informative and inspirational.