Some enterprising user of the Mob-Rule.com county counting site has created a map that colors in every county in the country touched by an interstate. Here’s the map, and here’s the page (to select by state). Major interstates have a color to them, while the rest are in default light blue. There’s one tiny omission: Putnam County IL, which is rather small, serves as the tip end of I-180.
Also, and perhaps more importantly, the website has implemented an API that projects county lines onto Google Maps! Plug a city in on the link, and it will then render a map page with county lines. Since this is an add-on, it can tax the computer and bandwidth if you try to see too many at once.
These can be handy for checking to make sure your map is accurate to your travels. However, if you’re just poking around here and there and want county lines (without Street View), Mapquest would still be the better site.