It does support addon applications, as well as replacing the built-in applications and associating file types. The default fbreader, for instance, has been extended
with accelerometer based page turning. The UI used is called inkview, and I've started slightly on trying to understand and document it.
I'm not sure just how much of the device is free software, but inkview itself appears proprietary. It's possible to run other frameworks, and some source, SDK and examples are at
pocketbook-free.sf.net.