Methods and systems for network-based or Internet-based software delivery are described. In one embodiment, an application program or software platform resides on a client. The program or platform is configured so that it is extensible based on software extensions that are deliverable over a network such as the Internet. Various extensions can be developed by third party developers for incorporation into the program or platform. In the described embodiment, extension files that comprise a software extension are hosted on an Internet server. Additional files are also hosted on the Internet server and describe aspects of the software extension. These additional files include files that describe a logical attachment of the extension to the program of platform, as well as files that describe the location of the extension files. Extensions are incorporated on a client by navigating to a particular Internet site through which the extensions can be accessed. The files describing the extension files are downloaded on the client. These files tell the client where the particular extension is to be plugged in, as well as where to find the appropriate extension files and how to download them. The extension files are then downloaded and incorporated into the program or platform.