Great educational websites (and an intro to Diigo)

As promised, I've compiled your suggested educational websites and posted them on Diigo, as well. Diigo, by the way, is a bit like delicious on steroids. It has a really straight-forward interface and allows you to highlight and annotate sections of a website (perfect for building web quests and helping direct research for your students). Feel free to check back on the zdneteducation page on Diigo for newly shared sites.

A quick note to folks who submitted links: I didn't post sites that have an associated fee. This isn't to say they don't have value, but I'm aiming to promote solid, free resources.