I'm a big fan of the AlphaSmart Dana. I learned about it a few years ago from good friend Kathy, and I wrote a decent chunk of Smart Home Hacks using hers. The Dana is a Palm OS-based laptop that has a decent built-in word processor, runs forever off regular AA batteries, and uploads what you've written to any Mac or PC by emulating a USB keyboard being typed on super-fast. Simple, but effective, and you don't need special drivers or fancy software to manage it.
The best thing about the Dana, aside from the battery life, is that it allows you to completely focus on what you're writing. There's no distracting email, web browser, or Twitter. It's also very lightweight, but tough (it's designed for kids), and a breeze to travel with. Lately, I've been traveling only with an iPhone and the Dana, leaving my MacBook Pro behind.
I'm glad to see that AlphaSmart is finally exploiting the utility of their products for professional writers. I spotted an ad in Writers Digest for their new Neo model, which is similar to my Dana but without the additional Palm apps which I've never used. Sight unseen, I have no problem recommending it for your consideration.