Microsoft's new MSN Virtual Earth is a pretty nice first try. It has some features that give Google Maps a run for its money, although some of MSN's space imagery is laughably out of date, and so far those photos only cover the United States anyway.
Here's one winner: you can zoom in a lot farther in some places with Virtual Earth than with Google Maps. Here, for example, is Virtual Earth's view of the Seattle's Bell Harbor Conference Center, home of the Gnomedex conference I attended last month. Here's as far as Google Maps goes.
Virtual Earth, like so many Microsoft products, looks cluttered, even at this early stage. But kudos to MSN for making their maps work in Safari on a Mac quite well—I wouldn't have expected that.