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London, England

Nicknames: Tech City, Silicon Roundabout.

Where: The majority of tech start-ups are concentrated around Shoreditch, east London. Other up-and-coming areas include the former Olympic Park.

Size: There are 34,400 digital technology businesses in London and around 155,600 digital technology employees, according to promotional group London & Partners.

Success stories: Two London start-ups have already been sold this year—DeepMind Technologies was bought by Google for $666 million and Zynga went to Natural Motion for $527 million.

Other start-ups: HAIL, a taxi-hailing app; Wonga, "payday" loans; and Raspberry Pi, a credit card-sized computer.

London Mayor Boris Johnson says: "There is nowhere to rival London for tech firms to thrive and grow—we have the talent, the investors and the entrepreneurial spirit."

Watch out for: London's inaugural Technology Week, June 16-20, 2014.

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Chris Ratcliffe | Bloomberg | Getty Images