KIDS Worldwide is one of the coolest and most sincere organizations out there that helps place international volunteers in over thirty children projects in developing countries around the world.
Kids Worldwide differs from other volunteer organizations in that they don't charge any money for their services. This means that volunteers only pay for their food and accommodation (no more than a couple hundred a month) directly to the project directors in native countries, thus directly benefiting the project they are going to help. There is no middle man and the cost is cheap to settle in a country for a month at a time to work with orphanages, charities, and schools.
Do your part. If you were thinking of actively getting involved with children causes, this is a great place to find areas that could really use someone like you.