As per Wikipedia, Microsoft SharePoint Designer is a WYSIWYG HTML editor and general web design program from Microsoft, replacing Microsoft Office FrontPage, and part of the SharePoint family of products.
It is part of the Microsoft Office 2007 family but not included in any of the Office 2007 suites. The name change, from FrontPage, reflects the emphasis on designing and customizing SharePoint websites. SharePoint Designer shares its HTML rendering engine with Microsoft Expression Web, and does not rely on Internet Explorer’s Trident engine which is less standards compliant.
Today we are excited to share with you some news about SharePoint Designer 2007. Starting now (April 2, 2009), SharePoint Designer 2007 will be available as a free download! We want more of you customizing SharePoint and feel that this a good way to put the tool in the hands of more people.