A few of yours plus:

ADMINISTRATION

• Active Admin - Simplify the way you manage your website and your models.

CSS

• bootstrap. Add twitter bootstrap.

HTML

• haml. Save time and gain clarity with your html code

• httparty. Makes http fun again! (not my words but pretty accurate)

API

• apipie-rails. Easily document your API

MISC

Of course don´t forget. Devise, sidekiq, stripe ...