Ars Technica calls Old Maps Online “the world’s single largest online collection of historical maps” but that’s not strictly the case. When I first read that I thought: what, bigger than Rumsey? Rumsey has 30,000 maps; Old Maps Online has 60,000. But Old Maps Online is a portal, not a collection: it has a damn slick timeline-and-map interface that brings up maps from the online collections of five institutions (so far), including the British Library, the National Library of Scotland, and yes, the David Rumsey Map Collection. At first glance it seems like a good place to start if you’re looking for a map of a specific time and place (as I have done on many occasions), and if they add more institutions to their database it will be even more useful.