[Update: YouTube released YouTube EDU this week]
While YouTube is great for entertaining videos, you have to dig pretty deep to find educational stuff (or know what you're looking for). Below is a short list of educational video sites:

Academic Earth - Selected lectures from top professors at Berkeley, Harvard, MIT, Princeton, Stanford, and Yale. Subjects range from Astronomy to Religion.

TED - "Inspired talks by the world's leading thinkers and doers"

While we're at it, here are two more cool links:

• Cosmos on Hulu.com - All of Carl Sagan's groundbreaking series
• Wikirank - Most popular and trending topics on Wikipedia right now