One thing that really struck me while reading this, though (and that also emerges in the comments to their post) is just how easy it is to do an equal amount of good through well-targeted financial donations. Though GiveWell caution against putting too much weight on rough quantitative estimates, their top-rated charities appear to work at around $50 per QALY (see, e.g., these unofficial deworming estimates, and their estimate of bednets as costing at the margin around $3200 per life saved). So perhaps we can think of each $700 (or £450) donated to GiveWell-recommended charities as a "kidney-equivalent donation".
And that's really cool. Because even if you're not willing to go under the knife to help others, so long as you're in stable employment you can probably pretty comfortably afford to make (multiple) kidney-equivalent donations each year. Merely giving 10% of your income, on a typical academic starting salary, might amount to roughly eight kidney-equivalents! (Maybe two or three times that for more senior professors...) This is an extraordinary thing to be able to achieve so easily. So I'm hopeful that, even if folks aren't persuaded to give up a kidney, many may be inspired to give more money once they realize how painless it can be to make a kidney-equivalent donation.
You might think of this proposal as the positive flip-side of Scott Siskind's rather dark proposal to start using "dead children" as a measure of currency (numbers edited to correct the outdated "exchange rate"):
"Doghouse that costs 250,000 pounds" might not carry the proper punch. "Doghouse that costs  dead children" does. Using dead children as a unit of currency carries a built-in awareness of opportunity costs. Yes, you can buy that doghouse, if you really think it's more important than spending that same money to save  Haitian kids' lives. Go on! Dogs watching plasma TV! That sounds adorable! [...] I'm not saying these people don't have a right to spend their presumably hard-earned money on whatever they want. Of course they have that right. I am just saying that if we took the simple common sense step of changing our monetary denomination from dollars to dead children, maybe they'd want something different.
P.S. Happy 5th birthday to Giving What We Can!