AFAIK Jimmy Carter supported decriminalizing marijuana during the 1976 campaign (or possibly only as a senator). That's all I really know though.
e: Not sure when he said this, but a quick google brings this up:
"Penalties against possession of a drug should not be more damaging to an individual than the use of the drug itself; and where they are, they should be changed. Nowhere is this more clear than in the laws against possession of marijuana in private for personal use... Therefore, I support legislation amending Federal law to eliminate all Federal criminal penalties for the possession of up to one ounce [28g] of marijuana."
e2: Looks like he said that during a message to Congress on August 2nd, 1977. So yes, he supported legalization as President.