Geekdad has a bunch of tips for using the round power-cells from a dead laptop battery. These cells, called "18650s," look like AA batteries, but have very different characteristics. Your laptop battery will contain lots of these (I have a mongo long-lived Thinkpad battery that I use while travelling with nine cells), and if any one of them dies, the whole laptop battery is rendered useless.
18650s are incredibly powerful and volatile, so be careful, because it's easy to blow 'em up or start a fire. That said, they're awful handy-dandy for providing a very long charge for very bright LED flashlights, or for powering your RC vehicles.
By the way, a good quality LED flashlight is incredibly bright. I tried to take some pictures and video to demonstrate just how bright, but you really have to see this with your own eyes, in person to appreciate it. And the LED is incredibly power-efficient, so it runs for a very, very long time on a single charge. It’s easy to see that the future of household lighting is not compact fluorescents, but LEDs...18650 Things To Do With An Old Laptop Battery (via Red Ferret)
In the video, I’m actually powering the Arduino as well as the motors, and I’m surprised it works. Motors tend to create a lot of electrical noise, and I’ve read about many other people who ran into trouble using a common power source for their Arduino and their motors. I presume I’d start seeing trouble if I was driving a heavier load than those little Lego motors.