Here's a cool tutorial regarding the circle of fifths I found over in the DB "Theory" forum for ppl who want to learn about key signatures and their relationships. Well written lessons about the characteristics of each key sig: what notes, what accidentals. Each lesson has a follow up test.
I jumped to the final test and got 40 points, the maximum. I'm bragging (sad isn't it?). Well, OK, I have been teaching this stuff for years...I'd better know it!