The Blue Book: A Consumer Guide to Virtual Worlds

Stephen Downes describes the resource better than I can:

"This is a very useful resource, not simply because it makes clear that 'virtual world' means much much more than 'Second Life', but also because of the glossary and categorization system that informs this list of 250 virtual worlds. Descriptions are brief (not surprisingly) and contain links to the world web site. The document is PDF, but you have to download a zip file and extract it."