Since today you are able to download WSS 4.0 / Microsoft SharePoint Foundation 2010 from Microsoft Download Center.
SharePoint Foundation 2010 is the new version of Microsoft Windows SharePoint Services. It is the essential solution for organizations that need a secure, manageable, web-based collaboration platform. SharePoint helps teams stay connected and productive by providing easy access to the people, documents, and information that they need to make well-informed decisions and get work done. Use SharePoint Foundation to coordinate schedules, organize documents, and participate in discussions through team workspaces, blogs, wikis, and document libraries on the platform that is the underlying infrastructure for SharePoint Server.
More details can be found here: