I just got back from the office blood drive, giving my periodic half-liter. It's a small thing that I can do to help, and I do it four or five times a year — I try to make it for every office blood drive, when I'm not out of town.
I'm told that my blood type, A-, is shared by about 5% of the population, so it's pretty uncommon (B- and AB- are rarer). And about 45% of the population can be given my blood. I'm also CMV-, which, I'm told, means that my blood can go to infants and immunocompromised patients. And so, throwing a pint at them every so often seems the least I can do. I can just replace it with a pint at the pub tonight.
Seriously, this is a very good thing to do. It takes little effort and time, and costs one nothing. Yet it can contribute to saving the life of someone who's in need of blood. They're careful about screening donors (and testing the blood) to keep the blood supply safe, and so not everyone is eligible to donate (if you've travelled to certain countries recently, been exposed to blood-borne diseases such as malaria, or lived in the UK during a certain period (and are therefore at risk of having eaten mad beef), they don't want your blood). But if you are eligible, please find a blood drive or donation center and do it. You'll have my thanks.
More importantly, you'll have the thanks of the people who benefit from your donation.