Here is a list of 15 popular GitHub repositories that every web developer should be familiar with:

1. Bootstrap

This is a front-end framework that provides a range of pre-designed components and layout options for building responsive, mobile-first websites.

2. React

This is a JavaScript library for building user interfaces, widely used for building single-page applications.

This is another popular JavaScript framework for building web applications. It is known for its simplicity and ease of use.

This is a comprehensive JavaScript framework for building web applications, developed and maintained by Google.

This is a runtime environment for executing JavaScript on the server side, allowing developers to build scalable network applications.

This is a popular web framework for Node.js, providing a range of features for building web and mobile applications.

This is a high-level Python web framework that encourages rapid development and clean, pragmatic design.

This is a lightweight Python web framework that is easy to get started with and suitable for small-scale web applications.

This is a popular web framework for the Ruby programming language, known for its convention over configuration approach.

This is a PHP web framework that provides a range of tools and features for building modern web applications.

This is a web framework for building web applications and APIs using the .NET framework.

This is a Java-based framework for building enterprise-grade applications and microservices.

This is an object-relational mapping (ORM) tool for Java that simplifies the process of interacting with databases in a Java application.

This is a static site generator written in Ruby, widely used for building blogs and other simple websites.

This is a static site generator built on React, providing a range of tools and plugins for building modern, performance-optimized websites.