Google Earth is now available and it's free. Only for Windows right now, though the download page says that they're working on Macintosh support.
[UPDATE: Ooo, there's even virtual sightseeing. {via}]
[UPDATE (6/29/05): Ooo, Google Maps now has an official API.]
[UPDATE (6/30/05): Google search for Google Maps API. It only shows up in the Sponsored Links, which I didn't even see for awhile.]
