Blood (mine, specifically)
Time to let the medicinal vampires do their thing!
A quick PSA:
If you're able to do so, please give blood! Blood supplies are generally lower than demand for blood products, so giving blood can very much save a life. So, please, check the eligibility requirements for your locale and, if you're eligible, go donate! Whole blood, red blood cells, platelets, or plasma, all are useful!
While I'm going on about this stuff, have you considered becoming a marrow donor? Marrow is also valuable for life and you don't have to be dead to donate. It's a bit more intense than blood donation (much more for proper marrow rather than peripheral stem cells), but you only give once a match is found. Definitely consider it if you haven't before.
And, finally, please register as an organ donor. In some countries, you might be by default, but a lot of countries require you to opt-in. Go check and register if you need to!