I donate blood. It takes less than 3 months to replenish, and I go again almost immediately.
I usually take a girl with me and have dinner with her afterwards. They're always like, "Wow, you're like... so amiable." Works every time...
But seriously, I do it because I can. I work right next to the clinic; it's also connected to a movie theater (For the Torontonians, it's the Varsity on Bloor). So I kill an evening donating a pack of blood and watching a flick. It takes about 8 blood packs (a week) to help someone with Leukemia. About 50 for someone in a car accident and in need of major surgery. There simply isn't enough blood going around, and ESPECIALLY from the youth (I'm 25, and I'm sometimes the youngest person at the clinic by decades). There's never any lines, because no one does it, so I'm out in 30 minutes. To reiterate: I do it so others can have a chance of life. Also, free Oreos.
They also check your blood for any abnormalities, so it's kinda like a free check up!
I also have a card on me that declares my organs free to be used, if I happen to keel over dead. I think that this should be law. If you''re dead, give up those organs.