The free version of JBoss Developer Studio is the standalone version which can be used with both community and productized versions of JBoss Application Server and related frameworks such as Hibernate, Seam, Drools, jbpm, etc. The difference between this distribution and JBoss Tools boils down to ease-of-installation and future updates. The product called JBoss Developer Studio Portfolio Edition is also available for 99$ and in addition to JBoss Developer Studio it gives access to JBoss EAP Server and additional downloads for development purposes of JBoss and Red Hat runtime distributions.
(Full Story: JBoss Developer Studio 4 is now available for free)