Looks like everyone is using Google Earth, but I'd also like to mention that ArcGIS Explorer might be worth a look for some people looking to do things that GE can't do. http://www.esri.com/software/arcgis/explorer/. ArcGIS Explorer is a free lightweight version of ArcGIS Desktop, which is one of the products made by Esri, the GIS (geographic information systems) company that has 90% of the world market. Here's a comparison on GE and AGX if you are interested in the details. http://www.scribd.com/doc/36390077/Google-Earth-and-Arcgis-Explorer.
Among other things, you can use a variety of data sources (doesn't have to be KML like Google Earth), so you can grab files from various GIS data clearinghouses like USGS topos, maps from NOAA, the EPA and state DEPs, and other things. They also have their own imagery, so you can get another version to look at. You can make custom maps, presentations, connect a GPS receiver, switch from 2D to 3D mode, have topos and a variety of basemaps, and add in other data layers from Esri's servers and much more. I use the pro version of this stuff every day, and the capabilities of true GIS far exceed free tools, but I just wanted to give an indication that there are other free things out there besides GE that do different stuff.
Among other things, you can use a variety of data sources (doesn't have to be KML like Google Earth), so you can grab files from various GIS data clearinghouses like USGS topos, maps from NOAA, the EPA and state DEPs, and other things. They also have their own imagery, so you can get another version to look at. You can make custom maps, presentations, connect a GPS receiver, switch from 2D to 3D mode, have topos and a variety of basemaps, and add in other data layers from Esri's servers and much more. I use the pro version of this stuff every day, and the capabilities of true GIS far exceed free tools, but I just wanted to give an indication that there are other free things out there besides GE that do different stuff.