I am glad you liked it!
He is most famous for the Fafhrd books that PY mentioned. But those are pure fantasy, which I don't enjoy as much as sf (besides I read them in translation, which was not too good). His most famous sf work is probably "Stranger" in which a moon-sized artificial sattelite suddenly appears next to earth, causing fear and destruction.
His stories are very good, so if you see a collection in a used books store - grab it. His most famous story (which won a bunch of awards) is "Gonna roll the bones".