I suspect advertising won't get much traction -- unless it's a product related to the subject of the book (I'm assuming nonfiction here).
Probably the best payoff from a freebie is getting the email from a downloader. That way you can form a relationship with the reader/customer over time. From the book perspective, you can send book announcements and special discount coupon codes. (I'm talking about outside amazon.com here).
Also you can drive traffic to your blog, which can in turn can lead to more ad possibilities and more things to sell.
I think the hardest thing for authors is author branding. A free ebook can help alleviate that issue -- especially if you can persuade them to sign up for an occasional newsletter.