Warning: this interactive map is a time sink! The map of contiguous US counties below allows the user to see what counties in the US share a name with any clicked county.
Mapbox posted this application in their developer examples (for Mapbox GL JS), and I thought it worth sharing. Mapbox does a very nice job with interactive maps, and their applications are fast and responsive. Hover over any county and immediately see what other counties in the US share a name with it, along with a number count and a sum population total. Zoom in and out with the mouse scroll or use keyboard '+' and '-' keys.
According to Wikipedia, the most common county names in the US are all named after US presidents: Washington, Jefferson, Jackson, or Lincoln. Franklin county comes in third (tied with Jackson and Lincoln), even though Benjamin Franklin was never president.
As new counties were formed in the olden days, the locals named them based on whimsy; whatever was in the public consciousness at the time. In this article from Oklahoma, the author lays out all 77 Oklahoman counties and where the names came from. The California Association of Counties does the same thing for that massive state's 58 counties. Wikipedia has origin names for each of the New York counties.