Is GlassFish really free to use?

Yes! The GlassFish Application Server is open source and totally free to use. Sun GlassFish Enterprise Server (previously named Sun Java System Application Server) is Sun's commercial version of GlassFish with a different installer and is also totally free to use.

So how does Sun make money if GlassFish and Sun's commercial offering are open source? In a word, services. Sun makes money selling services under the Sun GlassFish Enterprise Server brand, namely support, consulting and training.

productizes open source.

If you plan to go into production and want to purchase support (or purchase support in the future), Sun recommends installing Sun GlassFish Enterprise Server on production systems. Alexis has an excellent writeup about the benefits of support. The most important points are timely and accurate responses to questions with an SLA, access to the sustaining branch, and indemnification. Indemnification is a benefit of the support contract, not the Sun GlassFish Enterprise Server bits themselves. Arun also writes about the benefits of subscription in 3 Steps for Successful GlassFish Deployment.