> “like Google Maps, but with a time slider.”

There isn't a time slider in the map view, but if historical imagery interests you Google has several cool options:

The desktop Google Earth client will show you historical top-down aerial imagery that you can scrub through with a time slider: https://support.google.com/earth/answer/148094?hl=en

You can access historical street view imagery through a time slider in the main Google Maps web interface: https://googleblog.blogspot.com/2014/04/go-back-in-time-with...

For large-scale or global changes, there's https://earthengine.google.com/timelapse/ which is essentially a zoomable video of the entire Earth over the past 30 years at ~30m resolution.

Before everybody had GPS in their pockets, my buddies and I kicked around an idea for a "used-to-be map." It would help you when someone giving directions said "turn left where the Piggly Wiggly used to be."

The Dutch land administration has build an interactive map of the Netherlands that go back 200 years, 1815 to now It's quite cool to see how Amsterdam has grown over time.

Check it out at http://topotijdreis.nl

The Swiss topographic office map has a similar feature "Journey through time": https://map.geo.admin.ch

Here zoomed in on one of the big glaciers, which shows how it retreated: https://map.geo.admin.ch/?topic=ech&lang=en&bgLayer=ch.swiss...

I had the pleasure of meeting the guy leading NYPL OSS initiative last year; he demoed this among other things.

Talked about tons of other things NYPL is doing 'under the radar' to embrace the technology, it was pretty impressive, to say the least.

Always good to see OSS contributions from a government subsidized establishment.

for global coverage http://www.oldmapsonline.org/ is still the best thing round.

See also http://www.davidrumsey.com/ for general searching of old maps.