At first glance, World Wind (from NASA) might appear to be a space simulator similar to Celestia. But World Wind doesn't simulate the galaxy, it only shows you the Earth, and it shows it to you in amazing detail. Using your mouse you can spin the globe around and view the "Blue Marble" visible-color representation of the Earth; in that regard, it resembles a computerized globe. Once you've found an area you want to look at, you can zoom in...and in...and in, using several different kinds of world satellite picture data. In many metropolitan areas in the US, you can zoom in close enough to see individual houses and cars on streets (not in real-time; the satellite data is several years old). For much of the rest of the world, you can zoom in close enough to see mountains, rivers, large roads, and sometimes even large buildings (from a vantage point of about 6000 feet up). World Wind is like a computerized version of a classroom globe that lets you find most any place on the planet and zoom in as if you were flying over it.
World Wind's "layers" allow you to choose which kind of satellite and other imagery you want to display on the virtual Earth globe. Several versions of the Land Sat 7 global survey images allow you to get close enough to recognize roads and mountains, while some US Geological Survey (USGS) data sets let you zoom in close enough (in the US) to recognize individual houses (with the Urban Area dataset letting you see individual cars driving on the roads, but only in a small set of US cities). You can turn longitude/latitude lines on or off, or show country/state boundaries for the entire world. A "Placenames" layer adds text labels to countries, cities, counties, and even individual neighborhood, building, and landmark names as you zoom in (helpful for orienting yourself when you're trying to find something specific). A fun "Flags of the World" layer identifies the nationality of various areas on the globe by their flag; clicking on a flag takes you to that country's entry in the CIA World Factbook. Elevation is also represented on the globe, so if you change the angle at which you are viewing the surface (using the right-mouse button), you can literally fly through a mountain range with the mountains rising on either side of you (to move forward, you must click on a point on the globe that's in front of you, or click and drag with the left-hand mouse; the keyboard commands for "flying" like this don't work very well in the version I was using).
The image data for the entire planet is so huge that it isn't possible to download it all. The base download of World Wind is the application itself and the high-resolution full-globe image that you see when you are zoomed out. As you zoom in with one of the satellite image layers activated, the images are downloaded from public servers and are automatically displayed on the globe. As you zoom in and move around, World Wind finds the images for the spot you're looking at and downloads them. That means you're going to want a high-speed Internet connection to make using World Wind any fun. I also wonder if, as World Wind becomes more popular, we'll have to deal with the servers themselves being slow from responding to so many requests for data. For now, on my cable Internet connection, getting imagery for new areas of the world takes only a couple of seconds. World Wind keeps local copies of the image data you've viewed (up to a limit specified in a configuration file in the World Wind installation folder) so that subsequent views of the same area don't need to be re-downloaded. According to the World Wind FAQ, the full data set for all the satellite imagery layers would require about 10 terabytes (10,000 GB). What can you say - the Earth is a big place.
There is a growing community of World Wind users who are figuring out what World Wind is actually useful for. At World Wind Central, you can browse "hotspots" found by others, and view and contribute to a Wiki (user-editable set of Web articles) about World Wind. NASA's own World Wind Forums provides lots of information about World Wind, what to expect from it in the future, and lets you ask or answer questions (or provide suggestions) about World Wind. People are discovering ways to add more image data and other features to World Wind, and the NASA forums provide lots of information for extending World Wind's capabilities. I only hope it will be a little easier to do so in future versions.
In the series of pictures that accompany this review, I've zoomed in from an altitude of about 13500 kilometers to about 400 meters above sea level, showing a clear view of one of the taller buildings in downtown Portland, Oregon, including cars parked around it.
I think that World Wind has the potential to be a useful platform for a huge amount of scientific and educational data (and, eventually, commercial data; I envision a day when companies will sell layers that can be added to World Wind, like real-time traffic data for your city or state). For now, it has some educational value as a "digital globe", but I can't really think of anything useful to do with it. But it is fun to use, and as it grows in popularity hopefully more people will add to it and turn it into something really amazing.