I jailbroke my kindle a while back and was super excited to do it, but then didn't really do anything past that. Besides de-amazoning it, I found no real value nor supplemental features that were actually worth while. That being said, I don't regret doing it, just kind of wish there was more.
I installed Syncthing on it to make syncing new books onto it much easier. That's the biggest and primary value I got from it.
Like what?