The most commonly referenced 'useful' Nazi medical research brought up is the Nazi's work on hypothermia and the human body. Nazi's would subject concentration camp inmates to freezing cold water and record how long it would take them to die, or what methods worked best in reviving the prisoner or reversing the effects of the hypothermia. That medical research is still used today.

More nutourious Nazi doctors such as Joseph Mengele are known for conducting seemingly random medical experiments, as they were just doing experiments, but not nessicarily looking for any particular outcome.

this article goes very in depth on this topic and should answer a lot of your questions.