I have all of 'Twas the Night Before Christmas memorized.
I started memorizing it when I was a teenager, and about this time every year I refresh my memory (I only had 41 of 56 lines this morning when I tried it cold).
Somehow I doubt anyone is going to stop me on the street or call me up and require me to recite it, but if they do, I'm all set. Although I might flub a couple of words, or mix up the line order a little.
Edit: Okay, so ourika did call me and ask me to recite it. She didn't really want to hear it though.