When iTunes "Shuffle" function just isn't cutting it, give Moody a try. The little iTunes add-on lets you tag your music with color-coded, mood-based tags. Then, when you want some music to match your mood, just pick a corresponding color. Or colors – Moody lets you mix moods from a broad palette.
Granted, it seems like a lot of work to go through a large library and tag each song based on what mood it reflects, but I like anything that uses creative visual and emotional references to help people make decisions. Computers are cold machines, and apps like this help humanize them.
I should also point out that Moodlogic does something similar, minus the visual flare.
