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).
As an FYI, the expected benefit here is largely dependent on your ethnicity, and to a lesser extent, age:
1. White Americans/Europeans have very strong chances of finding a match (or many matches), so registering with that background means that you're likely less unique to the existing donor pool. Replaceability can often be high with this sort of background.
2. Those with other ethnic backgrounds can have much more difficulty finding matches. The less common your background (e.g., if you are mixed race, or from a less common background as found in the US, UK, and Germany (the countries which have the most donors)), the more likely it is that you'll be providing a donor where there is not otherwise a good candidate to be found, and thus actually sort of 'saving a life'.
3. Males under the age of 26 are typically considered the best donors, so you should more strongly consider this if you're a young, and/or male (although certainly many donors are used that don't match that criteria). If you're over 65, have a blood disorder, have received radiation, etc. you'll likely be told you're ineligible to donate to non-relatives.