Red Cross. But please - don't ever think that donating money is enough. You should always dedicate your time and effort to something, whether or not you have money to donate.
For instance, donate blood every chance you can (every 8-10 weeks). You help someone out and still get the satisfaction of free cookies.
I'm a big advocate of donating blood - I ran a blood drive at my high school once and helped to run it two other times. There's nothing more satisfying than finding out how many lives you saved in just a few hours.