It's still in development, but I'd be remiss if I didn't plug Epoch History: https://twitter.com/EpHisTech. It's an app that uses your GPS to lead history-oriented urban and rural tours. It may also have podcasts about particular topics. The best (and hopefully not worst) part is that it will be partially crowdsourced, in that users can label places with their own historical findings and lore, customize tour routes, and so on. It's shaping up to be a very cool project! They had a booth at Jacksonville's OneSpark crowdfunding event earlier this month. At launch it will be exclusive to that city but the creators are confident they can incorporate other locales soon after.

I've found this scanning app to be incredibly useful for archival and library research: CamScanner Free. It allows you to turn photos taken by your phone/ipad into a pdf. It will clean them up (straighten them, correct brightness and contrast, crop) and has a basic ocr function that kind of works. It integrates with dropbox and box.com, so it's easy to upload finished pdfs to the cloud so you don't fill up your phone on an especially fruitful archive visit. I find it to be much more convenient to use than trying to sort through hundreds of jpgs when I get home.

Google's field trip often has local (to your location) history

Quist is a gay history app I have on my phone.