There are many servlet engines these days. Here is a partial list:
This one is probably the most used. It is an excellent open-source servlet implementation brought to you by the apache project. Besides a stand-alone version, it can be integrated into a number of third party web-servers. To my way of thinking, unless you need a broad J2EE featureset, Tomcat leads the pack. http://jakarta.apache.org
WebLogic / WebSphere:
Both of these enterprise-level servers contain servlet/jsp engines as well as J2EE: EJB, JTS, JMS, etc. If you are using EJB, you will get a significant performance advantage by using their servlets which will access the EJBs in-process. These two come with lots of features and a big price tag.
Orion is similar to WebLogic and WebSphere in so far as it is a complete J2EE implementation. It is not as complete an enterprise solution but comes much cheaper.
A few years ago Allaire bought out the makers of JRun. Formerly just a servlet engine, the Enterprise version of JRun now claims full J2EE cerification.
Back before the Allaire buyout, ServletExec was a major competitor for JRun. ServletExec has traditionally been a solid product, although it doesn't offer the breadth of functionality that some of the others have.
This was first published in October 2001