Also known as "5'-adenosylcobalamine" for obvious reasons :-). It is essential for making the amino acid methionine, among other things. It is a peculiar compound that was one of the first complex organic structures crystallised. The crystallographer was the famous Dorothy Crowfoot Hodgkin who also produced many protein structures, as well as her more famous role in the determination of DNA structure.

An unusual cobalt-porphyrin compound, it is used quite a lot by methanogenic bacteria (or, rather, analagous cofactors are used) to do tricky carbon bond chemistry. Said to be the only C-M bond in biology, it is also useful for isomerisation.