I spent a little time searching through the Amazon Appstore for eBook reader apps. I have no idea whether Amazon will allow any of these to be installed on the Kindle Fire, other than the Kindle app, but it will be great if they do. Here is the list of apps I've located so far. I left out any minor players.
- Aldiko
- Djenne Book Reader
- gBook eReader
- Gutenberg eReader
- Kindle
- Kobo
- Mantano
- Moon+ Reader