For Kindle, the official
Time Limited Offers auto-generated listing will be of help, as will doing an 0.00 search on EReaderIQ as long as you exclude non-English and public domain books and stuff with a date between March-April 2011 when a huge slew of untagged public domain freebies got added. Sort by Popularity/Bestselling to generally get the latest stuff on the last couple of pages. Same parameters will work for the eReaderIQ UK search to find UK-only freebies, though they're a lot more tedious to slog through results for.
The
AddAll free listing for Amazon eBooks will generally have the latest Kindle freebies by date (although it'll omit any repeat freebies you might have missed). And their Sony and B&N pages are totally useless, though sometimes they catch something useful at Harlequin.
For B&N, volunteer posters actually keep a long-running thread which people regularly contribute to on the B&N discussions forums:
The Free NOOKbooks Summary Thread. Go to the latest posts and work backwards and some kind people generally compile of list of what's still free every couple of days and warn you of expired titles.
Inkmesh is sometimes useful for turning up new Sony and B&N and sometimes Kobo freebies.
For all the stores, sometimes checking to see if a book that's free in one store is free in any of the others will work, especially with certain publishers who are pretty generous about spreading their freebies around.
And if you've an interest in Christian Fiction, then Christian Books has a
page which keeps all their freebies, some of which end up being exclusive to them (or to show up before the other stores' listings do).
A few places do a regular monthly freebie deal, such as the
University of Chicago Press (academic usually non-fiction) and
Phoenix Pick Press (sf/fantasy; sign up for their newsletter to get the monthly coupon code) and some small specialty stores such as AllRomance and Rainbow eBooks sometimes do sporadic freebie giveaways which they usually tell you about in their newsletters as well.
The bargain deals may get posted less visibly, but in the past year-and-a-half since I've joined, MR has had plenty of people share the freebies and coupon codes around without any compensation beyond maybe getting some karma, and I'm sure that'll keep up even if the affiliate link incentive goes away.