Here's a post for noting those things which are being offered on special promo for just one day only which you'll have to check the respective sites daily for, rather than the stuff which will be presumably free/on sale for all of Read an eBook Week.
Central Avenue publishing is intending to rotate their freebie selection each day. Here is the
link to the dedicated promo page, where the 3 offerings for March 4th are for an urban fantasy, an historical detective mystery, and some sort of litfic/fantasy. Choice of ePub and Mobi direct download.
Untreed Reads is also offering a rotating daily selection and lists and links all of theirs
directly on the frontpage. You'll need to sign up for an account with them (no payment info required), and you get a multi-format DRM-free download. Today's title is a post-modern medical satire.
Dorrance,
Rose Dog, and
Red Lead are all very similar and I suspect they're all imprints of the same outlet. Once again this year, they are offering a daily freebie which you will have to sign up for every time. It's usually a PDF download and there's no indication (yet) as to what the actual free books are in advance of your submitting your info, but a note on the frontpages of each site just below the freebie-getting link notes that they'll be rotated at 8am EST each day.
The Marion Zimmer Bradley Literary Works Trust says they'll be offering freebies both
directly via their site and also on Amazon (presumably as part of the KDP Select program, which means they'll have a max of 5 days free). The first one is already up and is a PDF short story by Elisabeth Waters. (ETA: Waters is also separately offering her own works
on her own site, and the first freebie is a different PDF short story. You'll have to check Amazon on your own for whatever's free there.)
I think that's everyone who's doing a daily limited freebie and all the rest are doing week-long.