The Harvard Geospatial Library is:
A collection of 6,871 worldwide and regional geographic data layers, scanned historic maps and associated descriptive information that can be searched mapped and downloaded for use for use with your GIS software.
A significant fraction of the data layers are listed as “Restricted” (e.g. ESRI data), meaning they can only be used by Harvard staff and students. Still, there’s a fair amount of freely-distributable data available, although it’s a pretty mixed bag of stuff. My attempts to use the Basic and Advanced search functions usually yielded few to no hits; better to click on the “Map Browse” tab and then pan/zoom the map to your area of interest; you’ll then get a list of up to 1000 datasets that are relevant to that area:
Click on a dataset to get more info, and if it’s public, you can save it to your desired data list by checking a box. Once you’ve selected all the datasets you want, you’ll need to submit a request to the server along with your email address; in a few minutes, you should get an email with a link to a zip file containing the data. “Subject Browse” has a large number of subject categories and is worth a look, but a lot of the categories have no data in them.
Vector and raster datasets are available; the latter includes a fair number of historic maps for cities, like this one of the 1893 Columbian Exposition in Chicago:
The image comes with a world file for georeferencing, but the projection data has to referenced from a cryptic XML file link in the email; seems like this should be included in the distribution file as well.