I gave blood today.
Something to keep in mind is that there are some folks out there who want to donate, but can't for one reason or another (I couldn't donate for a few years after I came back from France on the off-chance that I had come in to contact with Mad Cow disease or something like that). So, all the more reason to go out there and do it if you are physically able. And, it just makes you feel good!
Call your local Red Cross office, or visit the American Red Cross online, for more information.
Go ahead, be a hero - save a life.