Board Memberships: Barr Foundation, Isabella Stewart Gardner Museum, WGBH, Museum of Fine Arts, Amherst College, North Bennet Street School, Belmont Hill School Recent Activity: Pledged $50 million to local climate-change prevention efforts in 2010 There are foundations based in Boston, and then there’s the Barr Foundation. The Barr is the biggest source of nonprofit funding in the region: In 2011, for example, the organization donated $54 million to 360 nonprofits, including $3 million to the Isabella Stewart Gardner Museum—part of Barbara Hostetter’s ongoing support of the museum (she played a key role in the capital campaign for the new Renzo Piano wing). And in 2010, the couple pledged $50 million to fighting climate change. With assets of more than $1.1 billion, the Barr seems poised to continue to give generously. Beyond merely writing checks, though, the Hostetters are influencing future generations of philanthropists with the Barr Fellowship program, which provides training and a three-month sabbatical for rising stars in the world of charity