Some time ago I read about a project in which bike commuters tracked their paths in order to map then on-line and have a place to see the most frequent routes in their city. Do you know this or other similar 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.
Garmin Connect also has a heat map functionality built in to their route planner.
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/
You're probably thinking of Rideable - Cycling Visualisations. They have videos showing commutes in Sydney, London and Bristol.
There are other projects such as the London Cycling Census. That compares different vehicle types (bicycles, cars, buses, lorries, etc).
More generally, The Guardian Datablog is a good source for this sort of information.
There's an android app mapmyride which sounds similar to what you want. It may also be on the iPhone