Case studies are specific examples of how GIS was used to solve a problem or made information sharing easier in a particular industry. The Library has a collection of books which illustrate a number of case studies. These case studies can also provide you with inspiration for your own GIS projects. This is not the complete list of case studies available, but those listed should help you with ideas.
Looking for statistics or data to actually map? Look here.
In addition to the books listed below, ESRI has published a series of booklets online to show best practices in different fields using ArcGIS.
Looking for GIS case studies on the Internet? Just do a search for "gis case studies." That will generate a long list. Then you need to decide what you want from the case study. That will help you narrow your search.