So many! But I'll restrict myself to seconding frik's mention of Stephen Donaldson (everything he's written), Cordial Jim's mention of Glen Cook (Cook is a go to guy when in doubt) and WeDealInLead's mention of Powers (Anubis Gates is in itself sooo good). Others not mentioned are Dorothy Dunnett (the queen of historical fiction to me), Jack Vance (the grandmaster, and my number one collecting target), and my newest favourite, K. J. Parker.