Gene Wolfe, who exhumes archaisms and obsolete words to make the settings of his science-fantasies (e.g., "Book of the New Sun") ring true

Have to second this - really amazing use of vocabulary, gives an exotic feel. As an example, I couldn't help but notice that he describes the "hylas singing in the trees at dusk," when he could just say "tree frog."

Another author whose use of words is admirable, and who gives one an education in quite a few interesting Scots words as well: Dorothy Dunnet, who wrote the Chronicles of Lymond. They're a six-book series of historical fiction set in Scotland in the 1500s, with a great hero/anti-hero, truly gripping suspense at times - good, solid stuff.