IcedTea-Web provides a Free Software web browser plugin running applets written in the Java programming language and an implementation of Java Web Start, originally based on the NetX project.

NetX allows Java applets and applications to be downloaded over the network, cached, and (by default) run in a secure sandbox environment. Subsequent runs of the applet download the latest version automatically. Update and security settings, among others, can be set using the itw-settings command.
IcedTea-Web also includes a plugin to enable Java applets within web browsers.

Features of NetX:

Modular: Easily add JNLP capabilities to an application.
Saves Memory: Launch programs in a shared JVM.
Fast startup: Runs applications from a cache for fast starting.
Security: Run any application in a sandbox or log its activities.
Auto-Update: Applications can auto-update without special code.
Network Deployment: Deploy to the internet, not with installers.
Open Source: GNU Lesser General Public License.

Visit the IcedTea project wiki or specifically the IcedTea-Web home pages for more information.
Help with common issues with IcedTea-Web can be found here.

Contributing:

A QuickStart guide for the IcedTea project is available on the wiki. Code style guidelines and Eclipse setup instructions for IcedTea-Web are available as well. Patches should be accompanied by unit tests and reproducers before being sent to the mailing list.