Well, there is his partner in his later years, Jane Lindskold.  She finished a couple of his books after he died.  You also might try Kage Baker's Company series.

I agree nobody is quite like Zelazny, alas, but here are a couple of people who come closer than most in one way or another:

Steven Brust.: He's written mostly fantasy, mainly one series about an assassin (it's cooler than it sounds) starting wtih "Jhereg." His feel is surprisingly similar to Zelazny's - the mixture of levity and poetry. Brust named his son Corwin, so he obviously feels the influence!

Elizabeth Willey: She wrote a 3-book series that is sort of a takeoff on Amber - very similar setup (multiple worlds, feuding family that has created them and can navigate them, etc.), though of course not really the same feel (they're more straight high fantasy, as you can tell from the titles). It' s not quite up to RZ at his best, but really pretty good. The books are "The Well-Favored Man," "A Sorcerer and a Gentleman," and "The Price of Blood and Honor." I don't think she's written anything since.

I once saw Zelazny read, BTW, around 1990, and I immediately thought, he's Dworkin! - he looked like a slightly mad wizard, aside from his lack of robe or pointy hat.