Linux server systems are widely used in various applications, and there are several popular system applications that are commonly employed in Linux server environments. Here are some of the popular Linux server system applications:

1. Apache HTTP Server: Apache is one of the most widely used web server software in the world. It provides a robust and scalable platform for hosting websites and serving web content. Apache supports various modules and configurations, making it highly flexible and customizable.
2. Nginx: Nginx is a lightweight and high-performance web server that excels at handling concurrent connections and serving static content. It is known for its efficiency and low memory footprint. Nginx is often used as a reverse proxy server or load balancer in front of other web server applications.
3. MySQL: MySQL is a popular open-source relational database management system (RDBMS) that is widely used for storing and managing structured data. It provides excellent performance, scalability, and reliability. MySQL is commonly used in web applications and other server-side software that require database functionality.
4. PostgreSQL: PostgreSQL is another powerful open-source relational database management system known for its advanced features, robustness, and standards compliance. It offers excellent data integrity, extensibility, and support for complex queries. PostgreSQL is widely used in enterprise-level applications and data-heavy environments.
5. BIND: BIND (Berkeley Internet Name Domain) is the most widely used DNS (Domain Name System) software. It provides domain name resolution services, allowing the translation of domain names to IP addresses and vice versa. BIND is used to set up and manage DNS servers for domain registration, hosting, and other network services.
6. OpenSSH: OpenSSH is a secure remote login protocol that provides encrypted communication sessions over a network. It allows secure remote administration and file transfer between systems. OpenSSH is widely used for secure remote access to Linux servers and secure file transfers.
7. ProFTPD: ProFTPD is a popular FTP (File Transfer Protocol) server software that enables secure file transfers between clients and servers. It supports various authentication methods, encryption options, and flexible configuration. ProFTPD is commonly used for hosting FTP services and managing file uploads and downloads.
8. Samba: Samba is an open-source software suite that provides file and print services for Windows-compatible clients in a Linux environment. It allows Linux servers to act as file servers, providing seamless integration with Windows-based networks.

These are just a few examples of the popular Linux server system applications. The choice of applications depends on the specific requirements and needs of your server environment and the services you intend to provide. Linux offers a wide range of powerful and flexible software options for building robust and reliable server systems.