Microsoft has rolled out a raft of 6 new fonts that are likely to start permeating web pages in the near future. They're nice fonts too - more modern and elegant than the dated-looking strokes of Verdana, Tahoma and Trebuchet. There's 3 serifs, a monospace and 2 sans and all start with a 'C' - Calibri, Constantia, Candara, Corbel, Cambria and Consolas (monospace).
More on them in the link below as well as a direct download link. As others have said - if you use CSS to spec them then you can start using them now by specifiying a fallback alternative family as second, third, twentieth choice for users that don't have them yet (it's likely these will start shipping for Mac with next gen versions of Office).
I have them on my Mac OSX system already and am beginning to find the odd site here and there that is already using them.
Font download here: http://www.techtoolblog.com/wp-content/uploads/VistFonts/VistaFonts.zip
Here they are: http://neosmart.net/blog/2006/a-comprehensive-look-at-the-new-microsoft-fonts/
Using them: http://neosmart.net/blog/2006/css-vistas-new-fonts/