In addition to the excellent and thorough writeups above, I would like to add a note about a less common aspect of blood donation: blood insurance. While this is probably not provided in many areas, at least one US state has such a program. The Medic Regional Blood Center in Tennessee automatically applies blood insurance to all donors. A single donation of whole blood nets you a year's coverage (two years for double red blood cells), and if you require blood in that time, Medic will provide it for free. Subsequent donations before the expiration of your blood insurance extend the coverage by another year, rather than resetting the year to the most recent donation.

You must wait eight weeks between whole blood donations, or sixteen weeks after a double red blood cell donation, which results in a constant potential blood insurance gain of 6.5 years/year. Of course, a year is burned in that time, so your potential profit every year is 5.5 years, making blood insurance an extremely efficient way of obtaining a free medical service for a very long period. The first time I gave blood was only about two years ago, and I already have blood insurance through 2016.

"Wait a minute!" you say. "Giving blood should be a purely altruistic endeavour. We shouldn't be discussing this in terms of personal benefit." You make a good point, but I make a better one: blood insurance covers your immediate family as well. This means that if you have a hemophiliac family member (as I do), you can indirectly provide blood for him, without worrying about conflicting types, on a schedule that's convenient for you, since you can give blood years before you redeem your credit.

Protecting yourself and your family certainly makes blood donation attractive, but your objections in the previous paragraph were well-founded; you should be giving blood as often as possible anyway, especially if you have type O. The possible gain for the recipient of your blood far outweighs the fifteen or twenty minutes it cost you to donate. The blood banks near me are frequently declaring emergency shortages, and sometimes they have fewer than 100 units on hand. The primary reason for this is that most people who are eligible simply don't think of donating blood. It's not a big deal until you need it, but when you do need it, you'll be glad someone was giving blood for you.