The recipe? A large public(ish) organisation (TfL) opens its data kimono to developers. Six days later, a group of geeks gather for a hack-day and a thing of beauty is born, the live tube map.
Have a look. It's rather addictive, like watching the download bar on a browser slowly creep across the screen (just me?). Congratulations to Matthew Somerville (@dracos) for hacking this together so quickly.