Christian publisher David C. Cook has been putting out two or three free books each week. They're sometimes free all week and sometimes just for a day or so (usually Tuesday). I've only been posting the ones that I find interesting, but others certainly have different tastes.
This link takes you to their freebies page.
If you missed a free book that you wanted, they have been putting the same books on sale for $1.99 or $2.99 for a few weeks after that.
This link is for their ebooks page that includes free, reduced price and new books.
From either link, clicking on each individual book takes you to an "about" page with links to a number of retailers. While Amazon and B&N have been participating for each book, some of the others (I'm looking at you, Kobo and Sony) have been spotty. If it's not free at one, try another. If you like PDFs, Google has been participating more lately and including PDF format as an option. If you prefer to support DRM-free, eChristian's David C. Cook books are watermarked, but otherwise DRM-free epub.
Today's freebies are
The Art of Caregiving by Michael S. Barry and
Reinventing Rachel by Alison Strobel (Lee Strobel's daughter). Both expire tonight (at around 2AM EDT by the looks of the timer).