The Romance Reader: A book review site, the thing that draws me there is that there is a special section called "Readers Helping Readers" which allows anyone to send in requests for help in finding out titles of books they have read and forgotten. All you need to do is provide any information you remember about the book and if anyone has read it before, you'll stand a chance of finding that elusive book again. Please note that they do not help you locate the book itself.
A Mystical Unicorn Online Used Bookstore: As the name said, an online used bookstore. The large database of Author Bibliographies provides me with a lot of help in tracing the backlist of authors and moreover, it is presented such that all sequels are grouped together so you know which you have to read in order.
Hard To Find Books
Barnes & Nobles
Romantic Times Magazine