Straight from Metafilter. Seemed worth passing along to our readers:
The Rijksmuseum in Amsterdam invites you to compare Caravaggio and Rembrandt. For an overview of Rembrandt's work here are Rembrandt van Rijn: Life and Work and A Web Catalogue of Rembrandt Paintings. For Caravaggio there's caravaggio.com which makes use of the Italian website Tutta l'opera del Caravaggio.