Patient H.M.:

Patient H.M., who suffered from intractable epileptic events, had a bilateral resection of portions of his medial temporal lobe in 1953. Losing portions of his hippocampi, amygdaloid nuclei, and entorhinal cortex, he has been unable to form long-term memories since the intervention, and thus is quite severely handicapped with anterograde amnesia. H.M. has been one of neuroscience's most well-known subjects for fifty years now, and a new study has been published to resolve the question of whether H.M.'s posterior parahippocampal cortex is responsible for compensatory spatial learning, given his other cortical deficits.

Mind Hacks also has an excellent post on this subject, which you can find here.

Here is the original 1957 study on H.M.

This NPR program discusses Patient H.M. and the history of his memory.