Yesterday, while feeling somewhat braindead, I started a public Zotero library of digitised historic map collections.
It's mostly Australian at the moment, and mostly the stuff I am always sending my family history students to, plus a few other places I use regularly for my own research, but I have grand intentions of adding to it over time – I keep a 'resources' folder in my own Zotero after all.
Feel free to shout your thoughts and additions at me here!
https://www.zotero.org/groups/5068384/map_collections_online/items/9GJ29ZGC/library