Wow, I'm surprised with all the netbook responses. I know they're nice and portable, but I find it hard to work with the small keyboards and screens, plus most I've tried feel a bit flimsy.
I use my trusty Lenovo Thinkpad T410. It's just the right mix of portability and power for me, and I get about 5 hours out of the upgraded nine cell battery. Plus, it's sturdy; I don't worry about damaging it when I'm on the go.