Giving Multiplier is a website that adds 3% to 30% on top of donations to any charity on condition that donors give a percentage (at least 10%) to an effective charity from their list of expert recommendations. It was created by the psychologists Lucius Caviola and Joshua Greene at Harvard University based on their research and is funded by Effective Altruism Funds.
Caviola, Lucius & Joshua D. Greene (2021) Boosting effective giving with bundling and donor coordination, PsyArXiv, Working Draft, March 22.
Giving Multiplier. Official website.