If you are a Google Earth user, you can view all kinds of fascinating free geo data available on the web. For example, this morning, I went exploring on the US Forest Service website and poked around the FSGeodata Clearinghouse. This collection offers interesting vector and raster-based mapping data for USFS lands in the United States, such as topographical maps. Some of the data in this site is KML-based.
The WMS feature of Google Earth also allows you to access publicly available geo data and import it seamlessly into Google Earth as an image overlay. Google Earth Pro users can import GIS data directly into the application.