Clarke Ashton Smith(CAS), along with Robert E. Howard and H.P. Lovecraft, was one of the founders of american pulp fantasy. He had a profound influence on the development of the fantasy genre and, while Gygax himself was not much affected by CAS, Tom Moldvay wrote the classic Castle Amber as an homage to Smith (much as The Lost City was an homage to Howard). Likewise, Jack Vance might be called his modern heir.
While my favorite writer in this group remains Fritz Leiber, I am fond of CAS, particularly his short stories. I have always felt his material is as inspiring for Sword & Sorcery as for Call of Cthulhu RPG horror. Xenopus in the J.E. Holmes "Blue Box" Sample Dungeon was inspired by CAS stories and that was my very first RPG adventure. I've been thinking of doing a CAS inspired Dungeon Starter for some time and while researching the idea came across the superb Eldritch Dark website (http://www.eldritchdark.com/) which had this little rpg gem (http://www.eldritchdark.com/files/articles/criticism/zothique-d20v1.pdf) posted on the site. It is is a d20 supplement describing the CAS Dying World of Zothique, simple to convert to Dungeon World.