Have you ever read a story, seen a piece of art, listened to an audio performance, or watched a film, and felt something? Have you ever thought, "Maybe I should tell the creator(s) what I think..." and then decided against it?
Well, creators crave feedback. To create something, put it out in the world, and then only hear crickets is one of the most discouraging things a person can experience.
So if the creative work moved you, and you want to experience more from the people who made it, TELL them. You don't have to impress them with your wit and wisdom. Just saying something to them in response to their work is enough. If you're afraid to speak up, check out this great FAN/FIC Magazine article, Why You Shouldn't be Afraid to Post Comments. The article is written with fanfic comments in mind, but the ideas apply to all creative works.
Don't know what to say? Check out Tabetharasa's great post, Comments and Criticism - Topics to Thaw Brain Freeze, for some ideas.
You also can't use "I don't know how to contact the creator" as an excuse anymore. We live in a more connected world now, where there's usually a way to contact the creator, even if it's just posting a tweet @addressed to their official Twitter feed. Sure, it might not feel like a personal connection, and it's a public comment, and it's limited to 140 characters, but it's something. And with feedback, something is better than nothing.
So make the world a better place today: if you're moved by a work, go lift the creator's spirits by giving them feedback!