Book Browser  at BarnesandNoble.com has a collection of printable Reading Group guides organized by subject.
BookMuse.com  - "Book discussions made smarter" Includes the following for each book covered: overview, topics for discussion, BookMuse commentary, literary criticism, author's bio, leader's tips, and reader reviews.
HarperCollins Reader  - Resources guides for fiction, non-fiction and perennial classics
Hachette Book Group USA  - Includes guides for selected books published by Hatchette, Little, Brown and Company, Back Bay Booksand other Hatchette subsidiaries.
Henry Holt Reading Group Guides  - Available online from Henry Holt/Owl Books/Picador
Penguin Putnam Reading Group  - Guides for fiction, non-fiction, classics and young readers.
Random House Book Group  - Their Book Group Corner has reader's companions from Bantam, Broadway, Dell and Doubleday.
Reading Group Choices  - "is the only comprehensive resource for discussion guides available for books by both large publishers & independent presses" Includes info on starting and running a group, recommendations and discussion guides.
ReadingGroupGuides.com  - "The online community for reading groups" has hundreds of reading group guides available and tips on what to do if there is no guide available for a particular book.
Simon & Shuster Reading Guides  includes over 800 reading guides.
Vintage/Anchor Reading Group Center  - includes tips on running a reading group, general discussion tips, guides to discussion particular books and suggestions for future reads.