...of your subject area...or so they say!
Shmoop. (Where do they come up with these names?!) A collection of study guides and teacher resources, mostly on literature and history, Schmoop provides student-friendly backgrounds on a variety of topics. I especially love their opening segment for each guide: "Why should I care?" which directly relates the issue to areas students consider relevant.
The guides are written by persons knowledgeable in the area (often Ph.D. candidates from Harvard, Stanford, etc.), and resources are copiously cited. In the music area, I took a look at the "You Can Call Me Al," guide, and found a whole new way of looking at the song (can you say, mid-life crisis, anyone?)
They are even starting to add teachers' editions to many of the guides.