Bookshop.org★★★★☆A good place to shop online for books and still support indie bookstores. They’ve started selling eBooks too, in standard ePub form. The reader app is a bit rough around the edges, but works well enough on a regular phone or tablet…just not on an e-ink touchscreen (yet?)
eBooks.com★★★★☆An eBook seller with some actual business ethics. No hardware (which similifies things), but they have an app for Android and iOS, and any DRM-free titles can be downloaded and read on just about anything.
Onyx BOOX Poke3★★★★☆I’ve been using the Poke3 as my main ebook reader for a couple of years now, mainly because it’s a convenient size, has a clear e-ink display, and can run the Android app for any eBook store.
Project Gutenberg★★★★★Predating the web itself, they’ve put together tens of thousands of ebooks from classics and other public domain sources in multiple formats from plaintext to ePub.
Standard Ebooks★★★★★Great source of classics and other public domain material, formatted and edited for maximum readability and compatibility.