Leiber's great - including the later Nehwon stories IMO, certainly up through Swords of Lankhmar. Personally I liked all the stories, but the ones set in Lankhmar are probably the best.
I think that the influence of Fritz Leiber's Lankmar stuff went a long way towards helping Gygax create a game that was not merely LoTR middle-earth role-playing. Without it I think the whole "D&D genre" would have been much the poorer.
Yeah, I doubt the whole "adventuring" business would have been the same without him.
That Brd/Bbn and Wiz/Rog sure set an interesting standard.
Not to mention the "thieves' guild" thing. Slightly silly, but fun
The first I ever heard of him was when the 2nd ed Legends & Lore book came out.
Unfortunately, after reading what little was there, his world seemed fairly bland to me so I've never bothered to look any of his works up.