...then mobile phones:

About an hour into a typical show on U2's Vertigo tour, Bono tells the crowd to hold up their mobile phones, in what has become the modern-day equivalent of flicking on a lighter. Instantly, thousands of blue-tinted screens illuminate the darkness as he marvels at the spectacle.
...
Then the band launches into the song "One," and Bono encourages the audience to use their phones to send a text message (also known as an SMS) to the one.org Web site, a sort of digital petition voicing support for poverty relief in Africa. Later, during the encore, the names of all who did so are scrolled on the same screen, and each receive a message of thanks from Bono on their phones.

This is one of the most visible examples of how the mobile phone is being used as a communication tool between artist and audience, turning the concert event into a much more interactive experience.

...via Smart Mobs