I was asking myself the same question the other day.

Well, for now if you're using/know Rails, maybe you could take a look at this: https://github.com/RailsApps/rails-recurly-subscription-saas

There are also other great Ruby gems/Rails projects that do Authentication (https://github.com/plataformatec/devise) , E-commerce (https://github.com/spree/spree), etc! That's why I love the Ruby community.