Tiger Woods, and apparently many other athletes, have tried platelet-rich plasma transfusion to accelerate their bodies' natural healing process to get themselves fit to play after injury sooner than normal. Platelet-rich plasma transfusion is used to heal tendon injuries, which are very common in both professional and amateur sports. The treatment involves collecting a patient's blood and removing his red and white blood cells so that only the plasma remains. This plasma, which is injected into the injury site, contains platelets, which release platelet derived growth factor (PDGF), which plays an important role in the natural healing process.
One of the reasons tendon injuries usually heal slowly is because they do not receive much blood; thus, very few platelets and PDGF ever reach the injured tendon. Injecting platelets into the injured tendon augments platelet delivery to the damaged tissue.
This article is a great introduction to platelet-rich plasma transfusion and also contains an interview with the physician who used it to treat Tiger Woods.
An important thing to note from the article is that we have not properly studied the effects of this therapy; thus, we cannot truly be sure that it actually works, nor can we be sure that it doesn't lead to some additional harm years after the injection is given. The article agrees that we need more research before we can state that platelet-rich plasma is a reasonable and effective treatment option for chronic tendon injuries.