This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
You don't need to pay for javadoc, it's a standard tool in the JDK.
I'm confused. First you ask for a tool and it looks like what you're looking for is javadoc, but then you point to a Sonar page, which is something completely different. Sonar is also open source and you don't need to pay for it.