Cosmos of the Ancients - Leucippus: Features Leucippus' thoughts on cosmology and the myths. By Stefan Stenudd.
Leucippus: Internet Encyclopedia of Philosophy article, detailing Leucippus' theory of atoms.
MacTutor History of Mathematics: Leucippus of Miletus: Biographical article by J.J. O'Connor and E.F. Robertson, with links to related topics.