Extensions are modular enhancements to Autodesk® software products and are available only to subcription members. They provide new functionality, are fully compatible with the base product, and are easy to learn. http://www.autodesk.com/extensions
Contrary to some of the rumors out there, extensions are still being delivered for AutoCAD. There is a AutoCAD 2004 extension now in beta and as a subsciption customer you are invited to enroll. This beta does require that you install the recently released AutoCAD 2004 Service Pack 1a.
Please note this beta should not to be used without careful consideration that these are unreleased beta products and are not fully supported and may contain software defects. As with any beta software always back up your data and previous files and exercise caution when in a production environment.
To enroll for the beta of this AutoCAD 2004 extension:
Please enter "Rx1" where the form asks for you beta code.