IBM's WebSphere, BEA Systems WebLogic, and Oracle Application Server are the big three in Java application servers, among the most successful commercial Java products.

But they're being overshadowed in one respect by a piece of free open source code, Tomcat, from the Apache Software Foundation. Tomcat is an application server designed specifically for Java applications, while the commercial products offer more general purpose, multi-language function. But when it comes to running the Servlets that an application uses to give the server Java commands, Tomcat is hard to beat.

"Tomcat is the clear leader for deploying enterprise Java Web applications," says Rod Johnson, CEO of SpringSource, the company that supports the open source Spring Framework for Java developers. Johnson's information is somewhat anecdotal. Spring users tell him they prefer the simple, high performance Tomcat for Web applications written in Java, and the Spring Framework supports Tomcat.

But there are figures that back him up. Tomcat isn't sold by anyone so it doesn't show up in market research by IDC based on product revenue share. It's available for free download from the Apache Software Foundation's Web site. In that respect, it's something like the Apache Web Server, which was never commercialized but became a market leader over commercial products.

Raven Zachary, open source analyst at the 451 Group, says Tomcat cites data from BZ Research, a software development research firm. The BZ Media unit conducted a Java survey in December 2006 and found Tomcat was being used by 64.3% of those surveyed. IBM's WebSphere was being used by 36.9%; Red Hat's JBoss, 32%; BEA Systems' WebLogic, 23.7%; and Oracle Application Server, 22.4%.

The adoption of Tomcat reflects the Java programmers' preference for lighter, simpler technologies. Tomcat is easily downloaded by developers and run in a Java development environment. Its specialty is running Java Servlets, the commands and short programs that govern Java Web server behavior, Zachary said.

In previous BZ Research Surveys, Tomcat hadn't been listed by enough respondents to show up as an application server player.

"Tomcat is fascinating because there's no big company trying to sell it. There hasn't been any marketing but it's gotten into the lead position simply by being relevant," says Johnson.

He thinks Tomcat has gotten stronger during the 12 months since the survey was undertaken, making Tomcat one of the hidden success stories of open source code in 2007.

Tomcat's numbers are helped by the fact that WebSphere, WebLogic, and JBoss will support Tomcat in their own operations. It's a developer platform that pairs up well with the commercial products when it comes time to deploy applications. But mainly, it's another sign of the desire for simpler methods in Java application development.