Product Fit - Measure product/market fit in app with your users.
YesInsights - Get Customer feedback with simple one-click or NPS surveys
Viable Fit - Measure and improve your product market fit
Wootric - Wootric is software that allows apps and websites to take customer satisfaction surveys so that you can properly gauge the popularity and success of your app through the eyes of the people using it. Read more about Wootric.
TinyNPS - Simple NPS feedback
FeedBear - The smartest way to manage feedback from your customers