Pocketbook is the main alternative. Hardware better than Kobo imo (e.g. their 6-inch eReader has buttons, Kobo's doesn't), and the software is even more open.
Pocketbook has far worse service and are not easily available in the US. Their use of flat bezels means that any small fall or bump will destroy the screen too.
I appreciate the aesthetic of flat bezels, but I find cases more annoying than bezels so I go for bezels to avoid needing to buy a new reader every 6 months to a year. My flat-bezeled Onyx Boox and Pocketbook readers have proven much more fragile than my fully-bezeled Kobos and old Kindle.
