Pretty much does what it says on the tin. No topography, no labels, no lakes. Just a big, zoomable map covered with all the roads in the lower 48. We think it's beautiful.
The map (which you must – MUST – check out in its full 10,000x6,000 hi-resolution glory) was created by redditor thechao some months back, but was recently resubmitted by one jason-samfield, after a similar road-only map of Oregon made some laps around the internet earlier this month. According to thechao, the map is based on the 2010 Tiger Dataset – a package of road data compiled by the U.S. Census Bereau:
It includes a large number of "products" (all free, all in the public domain), including very high resolution GIS data of roads, political boundaries, school boundaries, etc.
The data I rendered is the county road data (called "all streets" or "all roads"). I basically created a huge picture, then had a program draw a line where the Tiger data said each road is. Downloading, manipulating, and rendering the data is nontrivial.
No kidding. It's also damn awesome to look at.