There are already some very useful projects and toolkits available for WSS3 and MOSS 2007.
While reviewing the Codeplex site I filtered on some of the most interesting ones for me now:

• WS UploadService - a custom web service for uploading files into SharePoint.
• SharePoint Learning Kit - for creating e-learning sites with support for the e-learning standards like SCORM 1.2 and SCORM 2004
• Community Kit for SharePoint
• SharePoint Custom Forms Authentication Feature - a feature for implementing forms based authentication

And this is just to begin! There are lots more and certainly more to come.