I've been on the donor-after-death list via my driver's license for a long time now. Recently, I found a Nat Geo article that talked about brain donations for science, so I'm now a brain donor, too.
You can (post-mortem, of course) donate your brain to help with advancements in medical science at the Brain Donor Project. https://braindonorproject.org/
You can also visit https://registerme.org/ to donate organs, eyes and tissues.
It's also important that, if you'd like to donate other parts of your body, to make sure that get that on your driver's license/state ID when you apply for it. Contact your dept/bureau of motor vehicles or go to their website for more info.
While ALIVE:
If you'd like to donate a kidney or part of a liver or other organ, contact your nearest transplant center. There are currently over 120,000 people waiting for kidneys.
109,301 people need a lifesaving organ transplant (total waiting list candidates). Of those, 68,177 people are active waiting list candidates. (data effective 8/23/20 at 12:41 PM according to UNOS: United Network for Organ Sharing (UNOS)). https://unos.org/data/transplant-trends/
To donate marrow, peripheral blood stem cells and umbilical cord blood for the treatment of cancers, go to https://bethematch.org/.
Umbilical cord blood: https://bethematch.org/transplant-basics/cord-blood-and-transplants/
Marrow: https://bethematch.org/transplant-basics/how-marrow-donation-works/
Peripheral blood stem cells: https://bethematch.org/support-the-cause/donate-bone-marrow/donation-process/donating-pbsc/
Donate your blood: https://www.redcross.org/give-blood.html
I've signed up to save lives and improve the lives of future humans through advances in medicine. Will you?