Here are many of the sites I use for listing days my book will be free via Amazon's Kindle Select program or for buying advertising space to promote sales of my self-published books.
This new site eBook Booster has you fill out your information about your free promotion only once and they will submit it to 45 sites.
The Author Marketing Club requires a membership, but it's a nice all in one place to connect to several different promo sites when you have a book going free.
The number one advertising site that has shown the best results for me is Digital Book Today. This site's owner Anthony Wessel is a great guy and a former brick and mortar bookseller who is making a brilliant transition to selling books online. I think he's a rising star, and I'm hitching along for the ride with my advertising dollars.
And some additional promotional sites:
Good Kindles (requires donation)
FreebooksHub (requires donation)
Be sure to check out the links in this blog post to find other sites to list free books.
We know today that customer reviews are selling products at least as well as critic reviews. Reviews posted on Amazon and GoodReads particularly are good for your sales. If you don't have many reviews, you should attempt to get some, however, I think it is important to do so ethically. You can go to certain forums and offer free copies of the book to anyone interested in reviewing it. There is a site called Book Rooster where you can pay to have your book distributed to reviewers - but they are quick to tell you that you are not buying a good review - only an honest one. I used them when I first launched Sea Bitch, my short story collection. I also went to sailing user groups and offered free books to anyone who would be willing to give me an honest review. Once your sales start rolling, reviews will start rolling in one their own.
Best of luck!