Absolutely!
- If you like interactive fiction, there's a Gargoyle port.
- SkipStone is an alternative web browser (Gecko/WebKit).
- KindleExplorer lets you manage your Kindle's files without having to hook it up to a computer.
- If you have an older Kindle that you don't use anymore, kindlevncviewer can turn it into an e-Ink monitor + keyboard. This works great with a Raspberry Pi.
Also, from my experience, KOReader handles PDFs and ePub files much better than any conversion software does. The cropping functionality is fantastic for reading papers. You can also control contrast, columns, margins, line spacing, and so on on-the-fly.