While the above ideas are all good ones, I would suggest that most people have stopped recommending the Planetbike Superflash in favor of the Cygolite Hotshot. They're about the same ($30) price point, but the hotshot is brighter, more durable, and rechargable by USB cable. I use both on my commuter bike, but the superflash has been demoted to my backup light.
Also of note, if you're using one of the $5 red blinkers that are so ubiquitous these days, you're not really lit up. Both the superflash and the hotshot put those things to shame. You could have ten $5 red blinkers on your bike and you'd be less visible than using just one of these rear lights. I really think they're worth the extra money.