Maps elsewhere: Historic overlays
Having spent a number of late nights a few weeks ago struggling with our Toronto historic map overlay (the breakthrough was realizing that it wasn't ever going to be perfect), I'm very impressed with the 120 historic overlays on map collector David Rumsey's site.
There are a number of problems with historic overlays, but one freebie, if I can call it that, is that the historic maps almost always seem to use a very simple Mercator projection which is compatible with the API at a local scale. More modern map files can use any number of different projection systems, and while map overlays can be rotated, resized, and stretched and tweaked to a certain extent, it's very difficult to make up for it being a different projection.
The image is from an 1815 map of Montreal.