Note: Be the Match is only available for people living in the United States.
Be the Match is a sign-up list for people who would be willing to donate life-saving bone marrow to a compatible patient. The way it works is that once you sign up, you are mailed a swab kit which you rub on the inside of your cheek and send back. Be the Match keeps your data on file, and whenever somebody needs a bone marrow transplant, they scan through their database to find compatible donors. If it's you, you get a call.
The sign-up process takes maybe an hour including the time spent mailing the package. Be the Match estimates that the entire donation process, if it ever occurs, takes 20-30 hours spread out over 4-6 weeks, including phone calls, medical appointments, and the donation itself.
Let's say the average person reading this post can make $25-$50 an hour. That means they are spending the dollar equivalent of $500-$1500 to save one life, in the event that they are called upon to donate. I'm relatively new here, but my understanding is that that's reasonably competitive. It also takes the form of hours instead of dollars, which might make it more viable for some folks (and maybe less viable for others).
You can sign up here. Happy donating!
Note: If you are 45 or older, then there is a $100 registration fee (otherwise, it's free).
Note #2: Down below, user HaukeHillebrandt has pointed out a reason why this might not be as great as it sounds, and user MichaelStJules has pointed out a similar reason (see third bullet point of the linked comment).
FWIW, I was a stem cell donor in March this year.
Here in Norway you can sign up after donating blood at least three times, they take a couple of extra blood samples for the stem cell registry and then you wait for the call. Most people on the list are never matched with anyone. I understand that the registry is an international collaboration, so the patient could have very well (most likely) been from a different country than Norway. I don't know where the patient lives that received my stem cells.
Here's a database of countries and agencies that take part in this international collaboration: https://share.wmda.info/display/WMDAREG/Database#/
I donated through blood, which means I was first treated with injections of growth factors in the four days leading ut to the blood harvesting to provoke the release of stem cells from the bone marrow into the blood. This induce flu like symptoms with neck and back pain as the bone marrow swells up. The first injection my doctor gave me, but the rest I took at home after observing how it was done. On the day of the harvest I went to the hospital and spent about 3h in a bed hooked up to an apheresis machine which takes my blood out one arm, separated out the stem cells and puts the rest of the blood in the other arm.
The day of the harvest was a Monday, and I started feeling sick (the induced pseudo-flu) Friday evening with significant back pain on Saturday, Sunday and Monday, but I was back to work the second half of Tuesday.
In total I think I spent a total of ~30 hours of productive hours (including Sat and Sun), while the Norwegian Bone Marrow Donor Registry refunded my workplace for the 15 hours I was on leave for this. I didn't pay anything in the end, nor got compensated anything else than my normal salary for the hours I was on leave. Some of this time I spent on low productive things like reading books and the rest I spend relaxing, playing computer games or sleeping.
I don't think this was the most effective thing I've done in my life, and I'm unsure it was even better than the expected value of my hours, but I'm confident that was higher than average.
Since I already was a blood donor it was very low effort to registrer for stem cell donation, and when I was a match, the relevant calculation on my part was the time I was going to spend, not the time all non-matched donors already had spent.
I also value that I learned some interesting things about stem cell treatment and stem cell donation in particular, and I enjoyed the experience and the story I can tell afterwards.