From their Website:

ONE is a grassroots campaign and advocacy organization backed by more than 2 million people who are committed to the fight against extreme poverty and preventable disease, particularly in Africa. Cofounded by Bono and other campaigners, ONE is nonpartisan and works closely with African policy makers and activists.

At ONE, we achieve change through advocacy. ONE works closely with policy experts, African leaders, and anti-poverty activists. We hold world leaders to account for the commitments they've made to fight extreme poverty, and we campaign for better development policies, more effective aid and trade reform. We also support greater democracy, accountability and transparency to ensure policies to beat poverty are implemented effectively.

Sponsored by K-mart & Motorola  (2001)

Heath was often involved in charities that helped children, even anonymously volunteering at Perth hospitals dressed as a clown or mime to entertain the terminally ill children.  The Toys for Tots Christmas drive in 2001 was just another example of his dedication to helping kids.

Before heading off to Chicago to complete The Dark Knight, Heath met with Captain Paul Watson of the Sea Shepherd Conservation Society.  Passionate about preserving the oceans and sea-life, Heath  joined their Advisory Board October 2007.

From their website:

Sea Shepherd's Mission Statement

Established in 1977, Sea Shepherd Conservation Society (SSCS) is an international non-profit, marine wildlife conservation organization. Our mission is to end the destruction of habitat and slaughter of wildlife in the world's oceans in order to conserve and protect ecosystems and species.

Sea Shepherd uses innovative direct-action tactics to investigate, document, and take action when necessary to expose and confront illegal activities on the high seas. By safeguarding the biodiversity of our delicately-balanced ocean ecosystems, Sea Shepherd works to ensure their survival for future generations.

Oxfam International's Mission Statement

Sponsored by Marie Claire AU & the Australian Conservation Foundation

From switching to green power to offsetting air travel with carbon credits, our celebrity supporters Cate Blanchett, Heath Ledger, Susan Sarandon, Rose Byrne, Dr David Suzuki and Selma Blair, have joined with marie claire to show just how easy it is to reduce carbon emissions.

Actor Heath Ledger, the Australian star of Brokeback Mountain and father of Matilda, leapt at the chance to join the campaign and draw attention to the issue of climate change.  He is passionate about the environment and is adjusting his lifestyle to reduce carbon emissions - even driving a hybrid car.  "I have a Toyota Prius," he explains.  "I care about our children's future."

Known for his generosity to struggling Australian actors, Heath was an Ambassador for the AustraliansInFilm organization. After the loss of their compatriot, AustraliansInFilm announced the establishment of the Heath Ledger Scholarship: 

"In honor of Australians in Film Ambassador Heath Ledger a scholarship fund has been established to capture Heath’s generous spirit and serve our mission to help and celebrate Australian actors. The Australians in Film Heath Ledger Scholarship will provide talented Australian actors in the early stage of their careers the chance to travel to the US to further their training and international acting opportunities."

From their website:

AustraliansInFilm was established in 2001 to create an environment where Australians and Americans can foster strong and long lasting relationships through Australian screen culture and the individuals who create it.

Based in Los Angeles, AustraliansInFilm hosts approximately 21 events a year for members and guests. Previous screenings have totalled over 140 feature and television films and current membership boasts more than 550.

Australians In Film Mission Statement:

• To celebrate and promote the work of Australian film and television
  makers.
• To create a Los Angeles-based networking forum for professionals working in the film, television and allied industries who are Australian or who have a close association with Australia.
• To increase the visibility of the Los Angeles-based Australian film and television community.
• To provide a vehicle for raising the profile of Australian film and television productions.

The AustraliansInFilm structure consists of a board of advisors, board of directors, administrator, ambassadors and general membership.

When Heath & Michelle moved into their Brooklyn neighborhood to settle down and raise their upcoming daughter, they both joined the Advisory Board of the Develop-Don't Destroy Brooklyn organization.  The current fight of DDDB is to oppose the abuse of eminent domain laws by a billionaire developer who has used his connections to force people out of their homes in order to build several hi-rise luxury complexes that will block Brooklyn's view of the Manhattan skyline.

Their goal is to promote development without ruining the neighborhood feel of their borough.

From their website:

DEVELOP DON'T DESTROY BROOKLYN (DDDB), is a volunteer-run and community-funded 501c3 non-profit corporation. DDDB was founded in February 2004.

DEVELOP DON'T DESTROY BROOKLYN leads a broad-based community coalition fighting for development that will unite our communities instead of dividing and destroying them. DDDB is opposed to Forest City Ratner's 8 million square foot "Atlantic Yards" development proposal for an arena and 16 high-rises in Prospect Heights and Park Slope, Brooklyn. The $4 billion project would use at least $1.6 billion in public money and would abuse the state's power of eminent domain–taking private property from one owner to give to a private entity for a private use, instead of a public use.

Our coalition consists of 21 community organizations and there are 51 community organizations formally aligned in opposition to the Ratner plan.

DDDB believes that New York City is always about change, but we ask the question: How do we, the people, want that change to occur? We want smart collaboration that creates decentralized, diverse, exciting urbanscapes that New York and Brooklyn can point to with pride. We do not accept the abuse of eminent domain. And we do not want top down, sweetheart mega-deals that give one real estate developer carte blanche.

DDDB supports growth at the MTA's Vanderbilt Rail Yards, with truly affordable housing and career creation; that development must respect the quality of life in surrounding neighborhoods, the well-being of existing businesses, the scale of local architecture and the health of nearby residents. Zoning changes MUST benefit residents, while allowing for new uses. There must be oversight by local officials from community boards to the City Council. And most importantly there must be genuine input from affected communities.

DDDB is fighting the Forest City Ratner plan so that we can have responsible development in our community. We are fighting and will continue to fight in the court of public opinion, political opinion and eventually in the courts of law.

DDDB is a volunteer-run organization. We have over 6,000 subscribers to our email newsletter and 7,000 petition signers. Over 800 volunteers have registered with DDDB to form our various teams, task-forces and committees and we have over 150 block captains. We have a 20 person volunteer legal team of local lawyers supplementing our retained attorneys.

We are funded entirely by individual donations from the community at large and through various fundraising events we and supporters have organized. We have the financial support of well over 4,000 individual donors, with a median donation of $55.