The most popular web server applications include:

1. Apache HTTP Server (Apache): Apache is one of the most widely used and historically popular web server applications. It is an open-source server software known for its stability, scalability, and extensive feature set. Apache supports various operating systems and is highly customizable through modules and configurations.
2. Nginx: Nginx (pronounced “engine-x”) is a lightweight, high-performance web server and reverse proxy server. It is designed to handle a large number of concurrent connections efficiently and can serve static and dynamic content with speed and reliability. Nginx is often chosen for its low memory footprint, event-driven architecture, and ability to handle high traffic loads.
3. Microsoft Internet Information Services (IIS): IIS is the web server application provided by Microsoft for Windows-based servers. It integrates well with other Microsoft products and technologies, making it a popular choice for hosting ASP.NET applications and websites built on Microsoft technologies. IIS offers robust features, security options, and integration with Windows Server components.
4. LiteSpeed Web Server: LiteSpeed is a high-performance web server that aims to provide compatibility with Apache configurations while delivering superior performance. It offers features such as caching, HTTP/2 support, and advanced security options. LiteSpeed is known for its scalability and is often used in high-traffic environments.
5. OpenLiteSpeed: OpenLiteSpeed is the open-source version of LiteSpeed Web Server. It offers similar features and performance benefits as LiteSpeed but with the advantage of being free and open-source. OpenLiteSpeed is popular among developers and small to medium-sized websites.
6. Gunicorn: Gunicorn (Green Unicorn) is a Python WSGI (Web Server Gateway Interface) HTTP server. It is commonly used to deploy Python web applications and frameworks, such as Django and Flask. Gunicorn focuses on simplicity, ease of use, and performance.
7. Node.js: While not a traditional web server application, Node.js is a popular JavaScript runtime built on the V8 engine. It enables developers to create highly scalable and real-time applications. With frameworks like Express.js, Node.js can function as a web server for serving JavaScript-based web applications.

It’s worth noting that the popularity of web server applications can vary depending on factors such as specific use cases, platform preferences, and evolving trends. It’s always advisable to consider the requirements and characteristics of your project when selecting a web server application.