Giving portal for artists??

DonorsChoose.org is a national giving portal for public school teachers across the country. They vet the proposals, promote you, give you the contributions to support your project (i.e. help us buy a new blackboard!). They support their own operation quite transparently it seems - big funders from Silicon Valley and some banks (uh, Lehman Brothers?) and an "opt out button" on their donation page that automatically gives 15-25% toward the operations of DonorsChoose. The very friendly guy I spoke at their office says that 90% of the donors give towards the operating costs. Is this because it is "opt out" and not "opt in"? Don't know but we all know that you gotta pay the rent and the salaries of the people running the company!! ANYONE KNOW OF SIMILAR SERVICES FOR ARTS ORGANIZATIONS?? any horror stories or fairy tales?