Humble Bundle
Bundle of Holding
Google Play Books is IMO the most consumer-friendly standard ebook service overall right now, but I admit I don't really give as much attention to DRM factors in this area. Reviews from customers are not super helpful there so check elsewhere if you are a review-optimizer. But the deals are honestly really amazing; search for "the" or some favorite topic, enable price drops, set max price to .99 and ask yourself how much you care about DRM in these cases :D (I do care myself but it's a nuanced sort of caring). Their comics bubble navigation (swipe up) is great too. They don't take the price-obscuring approach Amazon does to upsell Kindle Unlimited or promote authors who pay more, or however Kindle's obscurantist search/browse is supposed to "work".
A weird issue with Google Play Books seems to be caching-related though...I often have to force quit and reload the app so I can view the items I just added to my wishlist xD
Project Gutenberg always...I have a daily excerpts page which draws from almost 500 of my favorites there (profile)
Oh and various other publishers like Evil Hat, Green Ronin, Steve Jackson Games, Troll Lord, Goodman Games, etc. Some of them host mind-blowing deals on their sites occasionally. IMO if you are a publisher you should always do this, it's one of the best ways to leverage being a small publisher in a big world.
Speaking of which, get on email newsletters for ebook publishers, they'll often include links to every service they sell on. If you like Star Trek books then get on Simon & Schuster's email list, this is like the best time in history to be a fan of Star Trek books... it's like a constant fire sale. Great strategy IMO.