Firebug
The Firebug extension for Mozilla Firefox allows the debugging, editing, and monitoring of any website's CSS, HTML, DOM, and JavaScript, and provides other Web development tools.
 
Not yet in possession of Firefox?
MySQL
MySQL is a relational database management system which has more than 6 million installations. MySQL stands for "My Structured Query Language". The program runs as a server providing multi-user access to a number of databases.
JBoss
JBoss Application Server (or JBoss AS) is a free software/open-source Java EE-based application server. Because it is Java-based, the JBoss application server operates cross-platform: usable on any operating system that Java supports. JBoss AS was developed by JBoss, now a division of Red Hat.