CSS

• autoprefixer-rails – automatically adds browser prefixes like -webkit and -moz to your CSS
• bourbon – helpful SASS mixins
• normalize-rails – CSS reset

JS

• gon – set JS vars in controller (e.g. current_user.id)
• turbolinks – fast front-end, without requiring complicated JS (e.g. React)

SEO

• meta-tags – set meta tags, open graph, etc
• sitemap_generator – generates sitemaps

Development

• rubocop – for yourself to write better Ruby
• better_errors – name speaks for itself
• letter_opener – whenever Rails sends an email, it will just show it in the browser
• dotenv-rails – use .env file for your ENV vars

Misc

• friendly_id – flexible way of generating slugs
• groupdate – group records by day/week/month, useful for analytics
• rack-attack – block spammers, scrapers, and other abusive clients
• shrine – extensible file upload gem
• simple_form – better way to generate HTML forms

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 ...

Debugging:
github.com/pry/pry - Pry gem, an alternative to the IRB shell.
github.com/deivid-rodriguez/b… - Byebug gem, you can pinpoint where the bug is in that specific code

STYLES:

rubygems.org/gems/bulma-rails --> add styles to your page, similar to twitter bootstrap
rubygems.org/gems/font-awesom… --> font awesome in embedded Ruby! So good for links
github.com/kaminari/kaminari --> for pagination, lets you break down your collection of items to be broken down

GRITTER GEM rubygems.org/gems/gritter --> users will have notification if they something to the app like sign out, post a question, add an answer. That black box you see that pops up on the upper tight. WIP.chat has this 😄