Tony Blair launched an online pledge to become a patron of a London sports club this week. BBC News talks to others who have used the internet as a way to make a promise.
Mr Blair famously did not have an e-mail address until 2003
The prime minister's offer of patronage does come with a catch. Like all vows made on the Pledgebank website, he is only bound to carry through his promise if others sign up to it.
He has asked for 100 supporters and so far nine have responded - including London Mayor Ken Livingstone and Cabinet minister Tessa Jowell.
Nonetheless, Mr Blair has become part of a growing internet phenomenon - using pledge websites as a campaigning tool, for publicity, or simply to come together with like-minded individuals.
THE PIANO TUNER
Andy Williamson is waiting for a kidney transplant
Andy Williamson, a jazz musician, got the idea for his pledge while visiting Guy's Hospital in London.
He used Pledgebank to promise to arrange a jazz performance for free in the hospital atrium - but only if five others would donate £10 to get the hospital's other piano tuned.
"I have a thing about musical instruments in public places," he explained. "If they're broken, I believe they should be fixed."
Andy, who has dialysis treatment at the hospital after discovering he had kidney failure, said he was "stunned" when enough people signed up.
Then, of course, he realised he would have to go through with his promise.
"I've started trying to fulfil my side of the bargain, and have been in touch with the relevant people at Guy's Hospital."
He said his call to the hospital had prompted them to get the instrument tuned, but the jazz concert was proving a little more complicated.
"I'm hoping to get the concert together some time in May. The organisation is getting a bit bogged down, as the person who can give formal approval is away."
THE FLIGHT CAMPAIGNER
John Valentine believes air travel is wrecking the planet
Instead of using other people's websites to do his campaigning, 58-year-old John Valentine from south-west London decided to set up his own.
Flightpledge.org.uk urges people to sign up to one of two pledges: The Gold Pledge, where signatories vow not to fly in the coming year except in emergencies; and the Silver Pledge, where they allow themselves two flights in the coming year.
He said: "There is a real problem because people feel really powerless."
He said aviation was destroying the planet, adding: "I'm trying to say to people that this decision will have more individual impact than anything else they could do."
John said he believed the "pledge concept" was a way for individuals to act in the face of "inadequate action by the government".
"But for me the only thing that is really significant is the decision not to fly," he said.
Despite only running for four weeks, he said the campaign has received the backing of over 600 people. One third have signed up for silver, two-thirds for gold.
"The numbers on the pledge are quite important. Once you get into the thousands, you can start to take more action."
His "fantasy scenario" would be for each individual to receive a flying allowance enabling them to travel a certain distance each year. They could then buy and sell allowances on the open market.
"But that needs to happen before the planet gets fried."
THE ID CARD ACTIVIST
Phil Booth organises a nationwide campaign against ID cards
Phil Booth is a sculptor, artist, lecturer and now national co-ordinator for the anti-ID cards campaign NO2ID.
Towards the end of last year, he asked 10,000 people on Pledgebank to refuse to register for an ID card and donate £10 to a legal defence fund.
More than 11,000 people signed up.
"At the beginning we wanted to give people some way in which they could do something positive before there was anything to resist - as ID cards haven't even been issued yet," he said.
"Pledgebank struck us as a good way to reach out to people - but the response was quite shocking. We set the pledge at about three months to give use time to promote it, but we broke our target within five weeks."
He said his pledge had asked for "pretty extreme" action.
"We were effectively asking people to sign up to something that could see them being put in jail - but once it got hold it seemed to have a cumulative effect.
"We have quite a number of lawyers who joined the pledge - as well as one sitting MP."
He said the ID cards campaign had now "taken over" his life and vowed the 11,000 pledges he had received would not be wasted.