Other
Theory on Tap: The Circle of Fifth
A nice site that breaks down into three sections: "what is the circle of fifths", "finding key signatures" and "guitar tuning and the circle of fifths."
Other
Ruby Cliff Computing: Learn Guitar Theory With Virtualoso Guitar
This is a fantastic site that teaches basic music reading, especially as related to the guitar. Although the site is a promotional software site, the trial version is excellent by itself.