I think I just mortgaged a few months of my life. Old Maps Online, launched last week, lets you browse a Google Maps-based interface for historical maps of places you like, anywhere in the world. A thrilling and dangerous proposition for us historical map obsessives.
So if I want, say, an old map of Emilia-Romagna (where I live) I can zoom into northern Italy, then choose from the list of maps on the right sidebar.
The concept is fantastically simple: link existing digital map collections in obscure libraries around the world with a simple interface that everyone knows how to use. The execution is great, too - the site is easy to use and feels intuitive. The project is a collaboration between The Great Britain Historical GIS Project based at The University of Portsmouth, UK and Klokan Technologies GmbH, Switzerland and funded by the UK government.
They plan to expand to other libraries' holdings, and are offering assistance in digitizing collections and georeferencing already-scanned maps. A crowdsourcing project to georeference maps in the British library using Klokan's cloud-hosted software took only 4 days. Keep your eyes out for more opportunities to help build this historical resource/incredible time suck. And pity me as I turn into a drooling map wretch.