Thanks to Michael Robinson for providing an introduction and link to David Rumsey's incredible online map archive, which focuses on maps of exploration. Michael writes
David Rumsey started digitalizing his map collection twenty years ago. He now has over 17,000 of them scanned, online, free of access to all. There are some amazing maps here, authored by Lewis and Clark, James Cook, and Alexander von Humboldt among others. You will need to download one of the specialized browsers on his site, but once this is set up you can view thousands of exploration maps in all of their hi-def grandeur and download them too.