I may have linked to this before, but I think that the contribution of Clark Ashton Smith to the Cthulhu Mythos is very important, and doesn't recieve the attention that many other of the contributors.
Clark Ashton Smith - The Eldritch Dark
"Clark Ashton Smith (1893-1961), perhaps best known today for his association with H.P Lovecraft and the Cthulhu Mythos, is in his own right a unique master of fantasy, horror and science-fiction. While he considered himself primarily a poet, and wrote over 700 poems and prose poems, it is for his short stories that he is best known today. Clark Ashton Smith was also a self-taught artist whose paintings, drawings and sculptures reflect the phantasmagoric worlds of his fiction.
"The Eldritch Dark is a site to facilitate both scholars and fans in their appreciation and study of Clark Ashton Smith and his works."
This sub-page outlines the cycles of Smith's writings.
These works make a great resource for Call of Cthulhu Keepers, particularly those running a Dreamlands or Cthulhu Dark Ages game. His work is well worth seeking out, and there are some very good resources on this page. I think that his work was a big influence on the movie The Brotherhood of the Wolf.
A quick update, I would like to thank John Rateliff for sending me a link to his very engaging article on Smith's writing, from the Wizards of the Coast website.