I am surprised to see that no one has yet mentioned Astonishing Swordsmen and Sorcerers of Hyperborea, an Old School game with roots sunk deeply into the fiction of CAS, HPL, and REH. It is a complete game with a full rule set (most similar to 1E AD&D and brown box D&D) as well as a setting - Hyperborea, a land of wizards, barbarians, Mythos monsters and strange gods drawn directly from the source material. The land itself is a flat plane encircled by seas ans the vortices of the North Wind.
The publisher is North Wind Adventures http://www.swordsmen-and-sorcerers.com/
A good forum can be found at the Original D&D Discussion boards.
I think it's not been mentioned because it's a fairly new and obscure game (i.e. from a small company). It certainly is based on CAS, HPL, and REH and looks like a fun game. However, their Hyperborea is not Smith's Hyperborea, which is a slight shame (presumably for licensing issues). It would, of course, be possible for someone to fairly easily use the system with a home made version of Smith's world (A Hyperborean Glossary
on Eldritch Dark web site summerises much of the Smith material, with some pastiche material) and find a map.
If you are into Old Time RPG this is certainly worth supporting, but it would have been nice if they could have gone for Smith's Hyperborea.
For additional reference, I found this blog for the game Mount Vhuurmithadon