Platelets!
Ads for blood donation used to say that the main reason people give for not having donated blood is that they haven't been asked. The point of those ads, of course, was to ask. I don't believe that giving blood really occurred to me as an option (despite those ads) until the September 11th attacks, when my school had a blood drive. Unfortunately, I was only 17 and I forgot to get my parents' permission. Once the immediate need was over, the idea of donating slipped away from my mind.
By the time hurricane Katrina hit in 2005, I had been injured and had my aversion to needles cured by the immersion therapy of having my parents less-than-artfully inject blood thinners into my waist twice a day for a few months. It turned out my blood type was a favorable O+, and after donating once they had my email to alert me to all further drives. I got into the habit of donating every two months, and was able to use this to assuage some of my guilt over the amount of medical resources I consume (to say nothing of guilt over not donating after 9/11).
After a while of donating regularly at the Red Cross's donation center in DC, they asked me if I'd like to donate platelets. I had never even heard about this - let alone been asked - but I tried it and for a few years now have been a very regular platelet donor there.
I want to recommend this to anyone who hasn't heard about it.
It is (and this was part of the appeal for me) more "hardcore" than normal donation - instead of taking 15 or so minutes once every two months as normal blood does, it takes over two hours once every two weeks. The process involves drawing your blood, separating out the platelets, and returning the remaining plasma to you (it is a single-use device, in case you were worried about that). When I tell people about this, they ask why I do it. It's sort of a strange question, really; why does anyone do anything charitable?
This is the kind of volunteering that I am well-equipped to handle, with a good blood type and a body that endures it well. Evidently platelets have a shelf life of about five days, so the need is constant - there's a small community of sorts of regular donors there. At this point, the nurses are even fairly emotionally invested in my frequently wacky dating life, and I look forward to recounting new stories every two weeks. And if you need more incentive, they give you a portable DVD player while you donate, and they have a wide selection of DVDs, including (for those around my age) the complete Firefly series.
Finally, I would like to add that the Red Cross has an app that tracks your donations and tells you how many lives you could theoretically have saved with your total donations. As of this writing, my number is 501. I have no idea how close that is to accurate, but if it is only ONE PERCENT of that, then it was all completely worth every moment.
This process may not be for everyone, but I assure you it is rewarding to know you've done something good that other people might find too difficult!