GlassFish is an open source application server project led by Sun Microsystems for the Java EE platform. The proprietary version is called Sun GlassFish Enterprise Server. GlassFish is free software, dual-licensed under two free software licences: the Common Development and Distribution License (CDDL) and the GNU General Public License (GPL) with the classpath exception.
GlassFish is based on source code released by Sun and Oracle Corporation's TopLink persistence system. It uses a derivative of Apache Tomcat as the servlet container for serving Web content, with an added component called Grizzly which uses Java NIO for scalability and speed.

Releases

The project was launched on 6 June 2005. On 4 May 2006, Project GlassFish released the first version that supports the Java EE 5 specification.
On 8 May 2007 Project SailFin was announced at JavaOne as a sub-project under Project GlassFish. Project SailFin aims to add Session Initiation Protocol (SIP) servlet functionality to GlassFish.
On 17 September 2007 the GlassFish community released version 2 (aka Sun Java System Application Server 9.1) with full enterprise clustering capabilities, Microsoft-interoperable Web Services.
On 21 January 2009 Sun Microsystems and the community released version GlassFish 2.1 (aka Sun GlassFish Enterprise Server 2.1) which serves as the basis for the Sailfin SIP AppServer project (aka Sun Communication Application Server).
On 10 December 2009 Glassfish v3 was released. Being the Java EE reference implementation, this was the first AS to completely implement Java EE 6. In this version GlassFish adds new features to ease migration from Tomcat to GlassFish. The other main new features are around modularity (GlassFish v3 Prelude already shipped with an Apache Felix OSGi runtime), startup time (a few seconds), deploy-on-change (provided by NetBeans and Eclipse plugins), and session preservation across redeployments.

(Taken From http://en.wikipedia.org/wiki/GlassFish on 3/22/2010)