I have seen a few implementations where the JBoss application server is used. Initially, the applications were hosted on licensed servers like IBM WebSphere and BEA WebLogic. But, due to the license cost, we advised our client to shift to an open source application server. With open source, we had a choice of going with Apache Tomcat and JBoss. We chose JBoss as it's far more scalable, secure and comes with support from the vendor.
The server supports Windows as well as Linux OS, which is an added advantage. JBoss seems to be a well-rounded application server in the market which supports scalability and is easy to set-up and to deploy.