Microsoft Office SharePoint Server (MOSS) 2007 is an industry leading platform for collaboration, but it's also a powerful application platform for .NET developers.
One of the new features of MOSS 2007 is the ability to include custom forms in workflows with Microsoft InfoPath, though to date you've needed MOSS 2007 Enterprise Edition or MOSS 2007 Forms Server to take advantage of this functionality.
Robert Shelton, Jr., a Software Development Evangelist on the Microsoft US Public Sector Developer & Platform Evangelism Team, put together a downloadable workshop series in which he demonstrates how to build a document workflow with custom ASP.NET forms for MOSS 2007 Standard or where ASP.NET forms are desired over InfoPath forms.
Be sure to check out the rest of Robert's blog for more useful information and training about SharePoint and .NET programming.
In addition, the SharePoint Server 2007 Developer Portal gives you access to information about building and deploying solutions for MOSS 2007, including articles, downloads, and community insight.