Once I got used to them, the easiest for me were the "advanced search" pages at
Barnes & Noble and
Amazon. The Amazon link is already for Kindle books, but you need to set the format at the B&N page to "NOOK book". Then enter a publisher and sort by price, low to high. The free ones will be at the top. You can then bookmark the page for each publisher that you want for easy reference. Here are example links for Agate Publishing at B&N and Amazon:
http://www.barnesandnoble.com/s/?sor...book&view=list
http://www.amazon.com/gp/search/ref=...ate&sort=price
Additionally, check the Amazon Kindle
Best Seller List. Click on "Top 100 Free" and then click your favorite category on the left. Most of the free books that get posted here make it pretty high on the list (and that's where I find a lot of mine), so even if that were the only place you looked, you wouldn't miss out on much.
I'm also a big fan of Google Books. There are all kinds of things that aren't popular enough to make it to Project Gutenberg, but that are still fun (dozens of cookbooks from the 18th and 19th centuries, for example). The
advanced search page isn't straightforward to find, so you might want to bookmark it. After your search, click on "Free Google Books" on the left. The big downside is that the OCR is sometimes pretty bad (especially for older books), but it's not a huge deal if you're OK reading PDF scans. And who knows? Your new favorite Brunswick Stew recipe might need
squirrel.