Contrail is a fun little gadget developed by Studio Gelardi to let bicyclists "mark" their bike paths. As more an more bikers ride on the same path, the contrail lines get brighter. Ostensibly, they say that the gadget allows other bicyclists the path that is safe to ride on but I'm sure that a big part of the appeal is the sheer fun in making roads look like Skittles rainbow.

Link - via Gadget Lab