Once in a while, you need to know which county a city is in (for example, because you need to call the county sheriff on a crime story), or conversely, which cities are in a particular county. One sometimes-handy tool for this is a Google maps mashup at maps.huge.info/county.htm. (Maps.huge.info also has a host of other useful data mapping tools: telephone area code boundaries, zip code boundaries, city boundaries, geocoders and reverse-geocoders) The only shortcoming to maps.huge.info’s county boundaries is that you must either already know the zip code of the county you’re looking for, or click in precisely the right location on a map of the U.S.
Another, often easier way to find your county is to use the somewhat under-loved map site MapQuest. Google Maps may have eaten a large portion of MapQuest’s lunch (along with breakfast and most of dinner), but MapQuest is still the only web map service I know of that shows county boundaries on all their maps at certain levels of magnification. If you don’t see the county lines, try zooming in one level at a time until they appear.