Cross Road Blues
This site is maintained by the estate belonging to son Claud Johnson, who finally got possession of Johnson's estate nearly 60 years after his father's death.
This straightforward bio includes a link to a Google map of Robert Johnson sites including death and burial sites (and yes, those are both plural as they are still unknown).
Advice from GuitarPlayer.com on how to imitate Johnson's deceptively complex guitar playing.
An interesting and authoritative website entirely devoted to the legend of Faust and its offshoots.