Content aside (I mean, those cheap mass market paperback editions from Signet and Dover Thrift offer most excellent content!), I'll focus solely on design (though obviously I only buy those books which I want to add to my collection, I have never purchased a book just because it looked "pretty"):
Everyman's Library Classics (those orange hardcovers)
Macmillan Collector's Library (those small, azure-blue hardcovers)
Zone Books (I don't have a lot of these, just those titles dealing with the Middle Ages, but they are beautifully produced)
New Directions (very minimalist, stylish design, for the most part)
New York Review of Books Classics (I am NEVER disappointed with their choice of cover art, the books are well-made and affordable, and they offer an amazing array of titles)
TASCHEN (again, I don't have a lot of these, but the production values are amazing)
Folio Society (but of course!)
Penguin Orange Collection (I wish they'd actually put out more titles in this series)
Centipede Press (only for those who like weird fiction, fantasy and horror; expensive but beautiful books)
Subterranean Press (same as Centipede Press above in terms of content)
Giramondo Publishing (I buy most of my Murnanes from them; their paperback editions are very handsome and they publish many other, lesser-known Australian authors)