Nice, thanks! Here's one for Firefox: https://addons.mozilla.org/en-US/firefox/addon/amazon-smiley/.

Thank you for these words.

For the time estimate question, this video seems to be saying that donation takes 3-4 hours and recovery is maybe 1 hour. The website gives more information:

Side effects may include back pain, fatigue, headache or bruising for a few days or weeks.

The counterfactual comparison is a good point. I couldn't find any information about it on the website, but Wikipedia reports:

Because the odds that two random individuals are HLA matched exceeds one in 20,000, a registry's success depends on a large number of volunteer donors.

Wikipedia also reports that the Be the Match registry has about 16 million members. This means that an average patient might have 800 matches, which bodes poorly for the marginal benefit of joining the list.

Good point. I'll add that to the post.

Holy crap, this is even more than I'd dared to hope for! I'm particularly excited to see your list of lists of important problems, both because I was dreading trying to figure out which problems were important myself and because the existence of all those lists is a good sign about the number of altruistic people in the world.

I'll read through these posts over the next couple of days/weeks. Thank you so much!