You might have to define "old". Anything pre 2000 could be considered "old" by today's standards. Disregarding that, I'd have to recommend What's Up Tiger Lily and Everything You Wanted to Know About six But Were Too Afraid to Ask, both by none other than Woody Allen. Both hilarious, and a tad weird but that's to be expected coming from Woody Allen.
I also want to recommend The Labyrinth to anyone that hasn't seen it, great movie. Watched it with my friend and I was skeptical at first, but this movie completely took me away. I wish I had seen it as a kid.
EDIT: Also, If...(1968) had an interesting plot with a tad bit of British 1960's beat culture thrown in for good measure. Risque ending by today's standards, but ends up making a good social commentary of school life. The ending to this movie really blew me away, though I'm not sure what the reaction to it was in it's time.
And One Flew Over The Cuckoo's Nest if you haven't seen it already.