Volunteering is part of our culture. From school days we were groomed to volunteer. The shramadanayas, projects of various clubs and societies in school, etc. are some examples. We gave our time and sweat towards simple projects such as cleaning church, spraying Mortein in hospital wards, etc.

A volunteer is someone who works for a community or for the benefit of environment primarily because they choose to do so. The word comes from France, and can also be translated as "will" (as in doing something out of ones own free will). Many serve through a non-profit organization – sometimes referred to as formal volunteering, but a significant number also serve less formally, either individually or as part of a group.

A volunteer work is not and should never be a required work.

A volunteer may or may not get paid or receive compensation for services rendered.

Volunteering comes in many forms: serving food at the local homeless shelter, providing computer technical support to a non-profit organization, acting in a leadership capacity on a charitable organization's board of directors or coordinating the emergency response in the case of a disaster. Around the world volunteer centers exist to support the voluntary sector and make a difference in the communities that they serve.

Some volunteer organizations allow room and board agreements, especially for those in leadership positions, or who do full-time rather than part-time volunteering.
- Wikipedia: http://en.wikipedia.org/wiki/Volunteer

About volunteering for NGOs.

I learned that some NGOs are mishandling funds. Some have budgeted and invoiced for expenses they have not incurred. Therefore, I would think twice about volunteering at an NGO. Especially, those who are not transparent in their activities and funds. It is possible that the top tiers of the NGOs would be stealing money that was supposed to be allocated for paid positions.

During the tsunami there were several instances of fund mismanagement by NGOs
Have a look at: http://www.google.lk/search?q=corruption+tsunami+ngo+sri+lanka

If I were to volunteer, I'd demand for transparency.