WebLogic, WebSphere, Tomcat, JBoss are all supported.
For a list of officially supported software for Oracle Web Center Sites(formerly FatWire Content Server), refer to this certification matrix : http://www.oracle.com/technetwork/middleware/webcenter/sites/owcs-11gr1certmatrix-1609746.xls
Prior to acquisition by Oracle, the JSK was bundled with JBoss and Tomcat server instances and it was possible to select one or the other as the deployment platform.
Now it only ships with Tomcat 6.