After creating a web application in SharePoint and then trying to configure a new Shared Service Provider (SSP), I was dismayed to find my web application list was empty. Because of this, there was no way to select my my web application and I was unable to create a new SSP.
After doing some digging I found out that the list will not show web applications that have a built-in account (such as Network Service) associated with the application pool assigned to the web application. After configuring the application pool associated with my web application to use my domain user account instead, the Web Application list on the Shared Services Provider screen was correctly showing my web applications. I was then able to successfully create my new SSP.