When I had my son, I had a can of formula from the hospital that I saved just in case. It didn't expire for quite awhile so by the time I had to supplement when he was 8 & 1/2 months old, I was able to use it. So, I would either save it just in case, if the expiry date wasn't for awhile, or donate it.
Domestic violence shelter for women. Take it to a police station and let them know that's where you'd like it to go. Usually they won't tell you where it's at for safety reasons, but they can take it there or have someone come and get it. Formula is in short supply at those places and those mamas really need the help.
Food pantries will take it too and with so many families loosing one or both incomes with this economy many can't afford the formula these days. I've also donated baby cereals and foods there. I second the domestic violence shelter idea too.