Just a list of Ruby gems that I find very useful for web development:
Devise - most commonly used gem when using Ruby on Rails. It provides an authentication module for a Rails application to allow login, registration, forget password, and other account-related features.
Draper - adds an object-oriented layer of presentation logic to your Rails application. Without Draper, this functionality might have been tangled up in procedural helpers or adding bulk to your models. With Draper decorators, you can wrap your models with presentation-related logic to organise - and test - this layer of your app much more effectively.
Cells - replaces partials and helpers with OOP view models, giving you proper encapsulation, inheritance, testability and a cleaner view architecture
FriendlyId - easily transform your URLs to much friendlier and easy to remember URLs for little to no code changes in your web application.
Kaminari - lets you to paginate anything from ActiveRecord relations to simple arrays using a clean, easy to use and simple scope-based API which is fully agnostic to whatever ORM or template engine you use.
Paperclip - Easy file attachment management for ActiveRecord
Pundit - provides a set of helpers which guide you in leveraging regular Ruby classes and object oriented design patterns to build a simple, robust and scaleable authorization system.
Simple Form - to easily build forms in Rails. It aims to be as flexible as possible while helping you with powerful components to create your forms.
Slim - a template language whose goal is reduce the syntax to the essential parts without becoming cryptic.
Sidekiq - A simple, efficient background processing for Ruby. It uses threads to handle many jobs at the same time in the same process.