A Few Tips:
As authors, we are always looking for new ways to get our books in front of a new audience! I have compiled a list of resources that will promote your free e-book for free! Some of them do offer paid promotions, but that is up to you and your relationship with the resources. I haven't tried any paid options yet, and still, see considerable numbers in downloads.
For each resource, there may be a few different rules, so before you submit your book, make sure it meets the requirements (ex. reviews, pages, subject, etc.) I've listed the requirements I've found but they may change!
I will add more as I find more over time!