If you want to be utilitarian about it, maximizing common good derived from a modest donation, then pick (a) the venture that will produce the greatest gain in

If you want to be utilitarian about it, maximizing common good derived from a modest donation, then pick (a) the venture that will produce the greatest gain in common good if implemented, and (b) an organization with a low administrative overhead that is in your opinion doing a good job of making progress towards (a). That is the 80/20 calculation. You'll never know if you got it right, but it has the best chance of doing something rather than nothing. For my money, (a) is ending degenerative aging by periodic repair of that damage that causes it, and (b) is the SENS Research Foundation. There is no other single cause of death and suffering as large as the biological wear and tear that causes degeneration and death in aging. There is no other organization I know of that will definitely put all donations to research, advocacy, and scientific organization relating to this task. There are other single-disease or single-field patient advocate and research organizations out there that do a little work relevant to age-related damage, but you've got no way to ensure a donation as small as a few thousand dollars will actually go to the aging-relevant 1% of what they do. Even cancer and stem cell fields are doing a large amount of work that has no bearing on the best path forward. In fact I'm personally donating $25K to the SENS Research Foundation this year, and running a matching campaign in search of others willing to do the same. There are other options for using modest amounts of money that are more of a lottery; use your money to persuade other people to give money, try to grow your money to give later, etc, etc. But these are much more unreliable, or for a few thousand dollars just not even worth trying versus outright donation to a cause.