I also give blood once a year. It's one of those winwin cases where there is a benefit for oneself as well as for others:

Blood donation

The studies related to this have some methodological issues but overall the effect size is so large, and the cost and risks so low, that it is worth inclusion. Several studies have indicated that, for men, regular blood donation results in a massive reduction in heart attack.[1][2][3] Other studies have found no such relation.[4] There are also additional health benefits to blood donation.[5] These are just some of the studies on this subject, but on balance after reviewing the evidence, I can say that donating blood once a year is almost certainly worth it if you're a man. Donating too often is probably bad for you though.

http://lesswrong.com/lw/jrt/lifestyle_interventions_to_increase_longevity/