As previously mentioned, I luvs me my Kindle. And I like to feed it regularly. To be honest, it gets fed new books WAY faster than I'll ever be able to read them. But that's beside the point. With the dubious ebook price increase rumors flying around and increased competition from Apple's iPad, I'm always on the look out for free/cheap books to stock my library. Thankfully there are some great resources out there. I hope to get these into my sidebar one of these days, for now it's easier to put them in a post.
Even if you don't have a Kindle, you may want to check these sites out. You can read the books on your computer (with Kindle for PC) or iPhone (with the app). Plus, many of the free books are only free for a very limited time (some only for one day). If you even think you'll want to read it someday and might maybe possibly be considering a Kindle of your very own, get the book while it's free. (Just last week I preordered a book for free and the next day Amazon raised the price to $8.49. Because I ordered during the free window, I got it for free.) Even if you never read the book, you aren't out any money.
Ok, so here we go....
Kindle Nation Daily
Stephen Windwalker's blog for all things Kindle. He wrote an excellent book on using your Kindle (so worth the buck it costs). He often posts a few times a day with Kindle related news and free/cheap book listings.
Books on the Knob
Mostly the same free books listed on KND but without the news and tech talk. Also lists books for other ereaders (like Sony and Nook) and freebie online games.
Source for free public domain books. Be sure to download the Magic Catalog to your Kindle. You can search from your Kindle and download ebooks directly to it. I've only scratched the surface of this service, but it does work. Magically.
One thing I find immensely frustrating about Amazon's Kindle store is it is very difficult to search for books by price. Jungle Search allows you to search on umpteen different levels (price, subject, keyword, reviews) and takes you right to Amazon with the results.
Another search engine that helps you find the best price across the web for ebooks. Search by title, author, keyword, whatever. You can narrow your search on many criteria as well...price, type of content (ebook, audiobook, etc), even by device.
I believe this site is mostly all indie published books. I've not checked it out much, but it looks like they've got a good selection of free/cheap books if you aren't looking for big name authors.
This store sells books in all eBook formats so you have to be careful to get Kindle compatible books. They've made it a bit easier by opening a Kindle dedicated site, but it doesn't offer a wishlist and some of the other features the main site does, so I tend to stick to the main site. They have lots of features I love including a wishlist, user ratings for books, book length references and tons of discounts. If you sign up for the mailing list you get a weekly newsletter with new releases tailored to your interest and most every weekend there are discount coupons. You can also join their Buywise club for even bigger discounts. I've been very happy with this source.
My final recommendation isn't a store or search engine. It is a software program to manage your ebook library and facilitate getting books onto your Kindle. If you get all your Kindle content from Amazon, this is probably unnecessary, but if you are like me and get content all over the place, Calibre is fantastic. You can take ebooks in any non-DRM restricted format and it will convert them to a Kindle friendly format and copy them to the device (via USB cable). I've done it with pdf files (no having to go through Amazon's conversion process) and prc files and it has worked flawlessly. It also has a news fetching utility so you can get newspapers and such without having to subscribe through Amazon (yes that means free). The trade off is your Kindle has to be connected to your computer and Calibre has to be running to download the news, whereas with the paid Amazon subscription it's delivered automatically via Whispernet. You can also grab single issues of newspapers, which is what I tend to do. Calibre also has a built in book reading utility so you can read your ebooks all from one reader. It's a pretty cool piece of software.