Visual Studio 2005 extensions for Windows SharePoint Services 3.0, Version 1.0 (sometimes called VSES for short) has been released! We've been waiting for this one for a while. We've only had a CTP build available since WSS went to RTM back in 11/2006.
The download became available on 3/15/2007: http://www.microsoft.com/downloads/details.aspx?familyid=19f21e5e-b715-4f0c-b959-8c6dcbdc1057&displaylang=en
These extenstions allow you to create a custom site, open that as a project in VS, add code behind, then save as a WSP. There are templates for Workflow actions, WebParts and Site Definitions. Of course, in order to use these extensions you have to be an admin on the server, running VS on the server because you write the site defs back to the server file system. Because of this, for most people what you'll want to do is install SharePoint and VSES onto a server image running in VirtualPC or Virtual Server, and dedicate one server image per developer so as to avoid conflicts at debug time.