There's a really fantastic blog featuring all of the animated shorts that have been Oscar nominated throughout the years. It's a treasure trove, really. I plan clicking each and every one of those video links and watching them to my heart's content. I don't recommend going to Matos' blog and checking out the nominated shorts unless you have nothing else you want to accomplish that day. :)
Anyway, while watching Flowers and Trees, I saw in the sidebar some of my absolute favorite Silly Symphony shorts. I've rediscovered so many of my favorite animations, I can hardly stand it! Here's a definite classic: The Ugly Duckling.