Kurzor, s.r.o. uses Auth0
Auth0 seems like the most viable way to have full-fledged user management in AWS without too much hassle. Aside of being paid, it offers many advantages, such as social network logins or handy administration interface.
Flux Work uses Auth0
Authentication as a service. Instead of rolling our own user authentication flow for yet another time, it's better to leave the security concern to a service specialized in this field.
hpneo uses Devise
Devise handles user registration with low effort. Also is configurable in so many levels, from custom views to create-your-own controllers.
rhnorment uses Devise
4 years of experience using Devise for User management, authentication, and authorization.