Urban rides map projects
Strava has a heatmap function you can enable on their route planner.  This will show the more frequently used streets and will vary intensity of color based on popularity.
http://www.strava.com/routes/
Garmin Connect also has a heat map functionality built in to their route planner.
http://connect.garmin.com/course/create
The garmin maps seem to be more effective in heavily used areas like city center and heavily trafficked trails, while strava seems to be a bit better at showing a bit more detail in both city and rural areas.
May also want to check out Race Shape's heat map, which seems to be a worldwide bike based heat map. Race shape pulls data from strava and ridewithgps.com to create these heat maps. http://raceshape.com/heatmap/
(Strava has taken over Raceshape and incorporated it in their premium product.)