Devise - Flexible and modular authentication solution.
OmniAuth - Flexible authentication solution for multi-provider auth (e.g. Sign in with GitHub or Google).
Pundit - Minimal authorization through OO design and pure Ruby classes.
job-iteration - Make background jobs interruptible and resumable by design.
countries - All sorts of useful information about every country packaged as convenient little country objects. It includes data from ISO 3166 (countries and states/subdivisions ), ISO 4217 (currency), and E.164 (phone numbers).