Application servers are usually essential software frameworks that are used by developers. There are many of these that are available. Here are some of the more important ones that are used popularly.
This is an application server authoured by JBoss and developed currently by Red Hat. It has been written in Java and runs on multiple platforms.
This is a free and open source web application server that has been written it Pytthon.
Apache Geronimo is an open source server runtime that integrates the best open source projects to create Java/OSGi server runtimes that meet the needs of enterprise developers and system administrators. Our most popular distribution is a fully certified Java EE 6 application server runtime.
This is an open source application server, which is also the official reference implementation for Servlets 3.0.
According to their website, this is “the fastest PHP server around”.
JOnAS is a leading edge Java EE 6 Web Profile certified Open Source OSGi Enterprise Server developed by Bull and OW2. (Official Website)
Jetty provides a Web server and javax.servlet container, plus support for SPDY, WebSocket, OSGi, JMX, JNDI, JAAS and many other integrations. These components are open source and available for commercial use and distribution. (Official Website)