2009 had the lowest mintage since the 50s, like 10% of a usual year. The recession meant that everybody was turning in their change jars, so their was low demand for new coins. So it might be a key date in the future. I find plenty, but I guess I'll keep saving them if people are willing to pay more than face for them.
1950-D is the key date, always a keeper, and very hard to find in circulation. 1938-D, 1938-S, 1939-D and 1939-S are also semi-key dates, and a few dates in the 40s and 50s are a bit scarce, and pretty much any with an S mintmark (either older or proofs.) Of course, keep all the silver war nickels with the large mintmark above Monticello on the reverse.
Don't keep 1964 nickels unless they're in EXCEPTIONAL condition. They were minted in HUGE numbers because of the coin shortage caused by silver hoarding, and they were even minted into 1965 and 1966. There will never be a shortage. Mintage numbers before '64 are a lot lower than after, so any of them might be worth keeping, though most are only worth face in circulated condition.