Google maps, of course...
I also use ACME Mapper:
https://mapper.acme.com/The topo layer often shows underground stuff like tunnels and drains.
And Wikimapia:
http://wikimapia.orgI would assume they use the same satellite images as Google, but the zoom feature seems to produce a much clearer image. Google maps (even in 2D) often show distortions brought on by how they stitch together satellite and flight photos.
Bridge Hunter has led me to some cool in-use and decommissioned bridges:
https://bridgehunter.com/I have also used Bridge Hunter to help me find Bandos. I am not good at just wandering. I need a destination. Load up some bridge locations in to Google Bitch, and tell her to avoid highways. The BEST way to view any state/country, and has led me to countless bandos just by happening by. So, the old bridges are definitely cool, but if I never make to the bridges I punched into Google Bitch, it means I found better stuff to explore!
History, history, history.... I am far from mastering public records and the library, but reading about the history of one location in great detail has led me other places by sheer co-inky-dink....
2 edits: Added another link, spelling, additional detail...