There are websites which will promote your book for free. The ones I've seen seem to have largely altruistic motives, as they are also self-published authors looking for exposure. Some use the promotion to get a kickback on sales from Amazon. Either way, it works for the author.
Here are some which you might like to try:
Anne's Book Corner - offers free book review exchange.
The Book Attic - offers free book review exchange.
Here are some more, added 17 June 2008:
The Story Behind the Book - send in the background story of your book and it will be posted in their blog.
Book Marketing Buzz - send in your request for free publicity.
Beyond the Books - submit a request to be interviewed.
Sheila Stewart's Book Cover Lovers - email Sheila from the link in the sidebar.