There are many others. Kobo for example doesn't lock you to anything and you can pretty much do whatever you want with the Linux on them.
Kobo is 10" max which is too little for me.
There are tons of e-reader out there, but if you aren't searching it's likely you will only know Kindle. Kobo is also very popular, but there're also PocketBook, Nook, Likebook, Onyx Boox, Bigme, etc.
Locked? Hardly. Connect the reader using a usb cable to a computer and it shows up as a drive you can drag file files on to.