Recently, I de-drm'd our family's collection of purchased Kindle e-books using skindle. In order to run them, you will need to install Python for Windows or Mac (which comes with Python, but apparently has older versions), or make sure you have packages for Python and its Tk binding installed on your Linux system.
The above tools work great on Linux, and should work fine on Mac OS X too as they are all Python-based. It also has handy plugins for calibre, which is open source software for reading and converting various eBook formats as well as syncing to many different kinds of eBook readers. It worked great for books downloaded on my Kindle 2. Instead it just uses the serial number from the Kindle they were purchased for to decode the books. One of them (called KindleBooks) allows you to remove the DRM without needing Kindle for PC. There is a set of Python scripts for removing DRM from several different eBook formats, including Kindle's.
