I would say that Geocaching would count as a prime resource. I have used this website for exactly this purpose in various cities. Geocaching is a game where locals hide something at very interesting locations for others to find. The game is to find as many "caches" as possible. The website typically caters locals, but that is what you/I want. You need a GPS unit. Any one suffice, but if you don't want to carry along all the papers with hidden caches, you might go for a subscription to geocaching.com and a more fancy GPS unit.
Having it as an app, can be specifically rewarding if you are traveling to a city, where a different language is spoken. You'll need google translate. The description might become a bit more complex to read after google translate, but coordinates don't need translations and even if you are not able to locate the cache, you are still directed to very interesting spot and google translate is often sufficient to get the main story.