And because it reaches so many people, because it is always with you, because it is cheap and sharable and easily repaired, the cellphone has opened a new frontier of global innovation.

Image IN DEMAND At an electronics market in Lagos, Nigeria, cellphones appear in great profusion, as they do throughout poorer lands. Credit... George Osodi/Associated Press

Babajob, in Bangalore, India, and Souktel, in the Palestinian territories, offer job-hunting services via text message. Souktel allows users without Internet or fancy phones to register by texting information about themselves. A user who then texts in “match me” will receive a listing of jobs suitable to her, including phone numbers to dial.

In Africa, the cellphone is giving birth to a new paradigm in money. Plastic cards have become the reigning instruments of payment in the West, but projects like PesaPal and M-Pesa in Kenya are working to make the cellphone the hub of personal finance. M-Pesa lets you convert cash into cellphone money at your local grocer, and this money can instantly be wired to anyone with a phone.

These efforts arise from a shortage of bank accounts in Africa. But they create the possibility of peer-to-peer finance that could be useful even in wealthy countries — for example, allowing small businesses in rural areas to collect money without credit-card systems.

I called Western Union, the Colorado-based money-transfer service, to ask if I could send money to a mobile phone. “Basically, we do not have that kind of option right now,” the agent told me. An American company, Obopay, does offer phone-to-phone payments. Its founder, Carol Realini, got the idea when volunteering in Africa.

The phone has also moved to the center of community life in many places. In Africa, urban churches record sermons with cellphones, then transmit them to villages to be replayed. In Iran and Moldova, phones helped to organize popular uprisings against authoritarian governments. In India, the cellphone is used in citizen election monitoring, and in equipping voters, via text message, with information on candidates’ incomes and criminal backgrounds.

Recognizing the role of cellphones in developing nations, the White House last year made a point of releasing President Obama’s speech to the Muslim world, in Cairo, in 13 languages over text message. It has made no similarly publicized gesture in the United States, even though not everyone has Internet access. (The administration proposes to remedy that by widening broadband access.)