USA: https://www.organdonor.gov/sign-up

Canada: https://www.canada.ca/en/health-canada/services/healthy-living/blood-organ-tissue-donation/organ-tissue.html

Germany: https://organspende-register.de/erklaerendenportal - New as of March 2024

India: https://notto.abdm.gov.in/register

UK: https://www.organdonation.nhs.uk/register-your-decision/donate (only Isle of Man is opt-in, rest is opt-out, so this might not be necessary, though it doesn't hurt; if you do nothing, your family will be asked to make the decision.)

Help needed to verify India is correct and to add more sites.