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