SharePoint Designer is Microsoft's WYSIWYG HTML editor and general web design program that replaced Microsoft FrontPage.
When SharePoint Designer replaced Front Page, Microsoft changed the HTML rendering engine to one that is more standards compliant - making it easier to design web pages that are readable in most browsers.
You can get more information and the download link at the SharePoint Designer home page.
There's also a Q and A from Microsoft, a letter to customers, and links to quite a bit of training material - web casts, demos, including a six part beginners series for beginners, sample pages and templates for use with SharePoint Designer.
Although they share a similar name (SharePoint), this is not the SharePoint Server program.
On a similar note, if you decide to download SharePoint Designer, you may also want to download Service Pack 1 (SP1) for SharePoint Designer 2007 (link below)
You'll need a Microsoft Passport, MSN Hotmail, MSN Messenger, or Windows Live ID account to download SharePoint Designer if you don't already have one. Registration is free if you decide to create an account.
Download Microsoft SharePoint Designer
SharePoint Designer Service Pack 1 (SP1)
Editors note: SharePoint is not really a replacement for FrontPage as described above. It is a tool specifically designed for the creation and maintenance of SharePoint based sites. - Gizmo