Most of their work is done by thousands of individuals who volunteer their time and energy to struggle for social justice, environmental protection and human rights.
Take the example of Amnesty International. It is almost entirely volunteer-based. Hundreds of thousands of individuals worldwide give up their leisure time to participate in Amnesty's letter-writing campaigns. Through their dedicated persistence, hundreds of political prisoners are saved from torture and execution, and thousands are released from detention.
This political activism is the hidden, unacknowledged face of volunteering.
It embodies the spirit of service, altruism, compassion and solidarity.
Volunteer campaigners have always played a crucial role in securing democracy and social rights. The Chartists and Suffragettes, for instance, were hugely successful volunteer social movements. Without their voluntary efforts, the denial of votes to women and working-class people would have taken much longer to remedy.
This highly politicised form of volunteering is a far cry from the public perception of voluntary endeavour. When most people think of volunteering, they think Florence Nightingale, genteel volunteers pursuing worthy, compassionate causes like caring for the sick and elderly.
It is time volunteering ditched its cuddly, kindly image and positively embraced the controversy that often characterises campaigns against injustice.
Volunteer effort has been central to every movement for human liberation.
None of the liberties we now take for granted such as freedom of speech and freedom of the press would have been possible without voluntary political activism.
Some activism such as the campaigns for old-age pensions, equal pay and socialised medicine, have brought massive improvements to the lives of millions of people.
Right now, all over the world, volunteer activists are struggling to end tyranny and injustice. When we think of volunteering, let's remember their efforts.
Peter Tatchell is a keynote speaker at the Institute for Advanced Volunteer Management conference on 2 November.