1)Curiosity is a good app for learning about interesting things, it has trivia which increases GK.
2)Travel triangle is a good app for finding package tours from local vendors of the place you want to visit and they provide cheaper tariffs compared to bigger names like SOTC.
3) Cred is good to pay credit card bills and get points at no extra cost which can be redeemed for goodies.
4) Safe in cloud is a good password manager and I find it better than LastPass.
5) Xe Currency is a good currency converter app, helpful interface and well updated every minute with the current exchange rates.
6) Locate my train is a good app to track live running status of trains and seat availability pnr etc, its better than where is my train.
7) Kayak and Ixigo are good to find cheap flight and hotels.
8) Videoder is the best video downloader for Youtube, sony liv, hotstar etc.
9) Walpy is the best for wallpapers.
Thats my short list of lesser-known gems from the app world.