Ardis Parshall at KeepaPitchinin announced today that the Church just began a new site for giving historical background to the D&C. No longer must we read the sections and wonder what happened to bring about such various and scattered revelations (sometimes many topics within one section). It is an ongoing project, so more lesson material will be added as they go along.
http://history.lds.org/series/
Thanks Ardis, and thanks to the LDS Church for such a great site!