Raspberry Pi isn't limited to a single server application, but the most popular and easy to install seems to be Apache. Apache has been around since 1995 and caught on the next year.

How to install Apache on Ubuntu Server. There are several ways you can install Apache on Ubuntu. If you simply want the basic server software, you can open a terminal and issue the command: ...

If you have a need for Java Servlets, you'll probably want to deploy Tomcat. Jack Wallen walks you through this process on the Ubuntu Server 16.04 platform.