swagger.io - Swagger is an open source RESTapi Documentation Tool.
UserGuiding - User onboarding made easy, for less. Create product walkthroughs in a couple of clicks, without breaking the bank.
Stripe - Online payment processing for internet businesses. Stripe is a suite of payment APIs that powers commerce for online businesses of all sizes. Use Stripe’s payment platform to accept and process payments online for easy-to-use commerce solutions.
Usetiful - Fight user churn with great user onboarding. Interactive product tours and smart tips significantly improve your user retention.
Stripe iOS SDK - Accept both Apple Pay and credit card payments easily on iOS
Userlane - Digital adoption platform with interactive guidance for software applications allowing anyone to understand and master any software from the get-go.