I haven't used Microsoft's map service http://local.live.com/ before, but I tried it today and it seems pretty good, at least for maps of the US. If you're using IE, you can also try out its 3D maps (works with IE6/7 on Windows SP2). Cooooool!
Google is of course not far behind - give the new Google Earth Beta 4 a try, which supports 3D layers and many more new features (pic). Download it now!