Two possibilities I have in mind:

The sony ebook reader: This uses a new technology for it’s screen that Sony calls eInk. It doesn’t need power to maintain the image. Supports PDF’s as well. It’s also an MP3 player.
Weights around 9 oz (255g) and is 1/2″ thick
Currently $279.99

Nokia N series, N700 or N800. (These run Linux) and are more than just a ebook reader
Currently N770: $150, N800