http://www.bookblog.net is an online book discussion group worth looking at. Each month one member chooses a book and leads the discussion. The site has an archive of past book discussions.
My question with online book groups: who brings the food and wine?
Reading Group Guides, very useful for book suggestions and discussion questions.
Fantastic Fiction list all books by authors in series and chronological order, invaluable site!
Vintage Reading Group Center: reading on a theme (instead of everyone reading the same title)