I find OSM (the website) is a great map. It's blazing fast (Google Maps aren't), and the color scheme is very clear. When I want to explore some random place on Earth (that uses a writing system I can actually read) I always use OSM.

Google Maps (or its local competitors) are great for spatial searches (ATMs in the area, shops in the area) and routing.

Finally, Wikimapia is great for virtual urban exploration. If you saw an abandoned building or warehouse, or found a suspicious bald spot in the middle of the forest when using OSM or Google Maps, the best place to learn about it is Wikimapia.