Peter Cameron has a nice collection of quotes related to mathematics sorted by subject (as requested) or rather keywords, like Inifnity, Axioms, Numbers but also some general sections. Not all of them are from mathematicians, but many are. He also links to some outher resources.

Peter Cameron has a nice collection of quotes related to mathematics (old version) sorted by subject (as requested) or rather keywords, like Infinity, Axioms, Numbers but also some general sections. Not all of them are from mathematicians, but many are. He also links to some outher resources.