Chargebee - Chargebee lets you manage subscriptions and payments at scale, handle custom recurring billing scenarios, reduce subscription churn and simplify accounting.

Anki - Anki is a program which makes remembering things easy. Because it's a lot more efficient than traditional study methods, you can either greatly decrease your time spent studying, or greatly increase the amount you learn.

Recurly - Subscription billing and recurring billing management. Recurly offers enterprise-class subscription billing for thousands of companies worldwide.

Quizlet - Quizlet allows you to review and create flashcards for a variety of subjects, such as math and reading.

Zuora - Zuora creates cloud-based software on a subscription basis that enables any company in any industry to successfully launch, manage, and transform into a subscription business.

Memrise - Learn a new language with games, humorous chatbots and over 30,000 native speaker videos.