James Cheshire, R user and lecturer at the UCL Centre for Advanced Spatial Analysis, created this map of the most trafficked cycle routes in London:
Click to enlarge, it's a gorgeous chart. The bicycle traffic data comes from London's public cycle-hire facility, which currently is only available in central London (which is why you don't see much activity in the outlying boroughs or south of the Thames). The map data comes from OpenStreetMap: the cycle routes are inferred from the pick-up and drop-off points of the bike rentals. James created the graphic from these data using the R language and the ggplot2 package, and kindly provided the R code and some tips to improve the appearance of similar charts. He also created a similar chart relating the bike journeys to atmospheric pollution.
By the way, ggplot2 package author Hadley Wickham will show this chart as an example in his webinar tomorrow, A Backstage Tour of ggplot2. You can register for the free webinar here, but note that we can only seat the first 1000 attendees, so be sure to sign in early if you want to watch the live presentation. (A recording of the webinar will be available 24 hours after the live event — all registrants will automatically receive a link to the recording.)
SpatialAnalysis.co.uk: Great Maps with ggplot2