Interactive NPS Calculator (by Delighted) - Calculate your NPS easily with this interactive free tool.
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
Growth Score - 40+ companies share their Net Promoter Score secrets!
Delighted + NomNom Integration - Turn your NPS responses into product insights
Customerville - Net Promoter Scores on steriods