The Subject Wikis Website is a website with a collection of subject wikis, primarily in mathematics. The website is still in an experimental stage. Check out:
The Subject Wikis Reference Guide: A good starting point for research. Has links to all subject wikis, and can be used as a starting point for lookup of terms.
The Subject Wikis Blog: A blog chronicling some of the ideas and choices behind the subject wikis.
For a more complete list of subject wikis, visit the Subject wikis reference guide.