There is a new website that not only lists all free e-books at Amazon (and contains direct links to each one), they offer a free service that allows you to enter Kindle titles you would like to purchase at a reduced price, and they will email you when/if the price drops at Amazon. Each title remains "active" for 180 days. You can also sign up to be emailed when Amazon adds new free content.
I recommend checking out this site. The owner told me last night he is eager for user feedback, so let him know what you like or don't like.
http://www.kindleiq.com