DonorsChoose.org allows you to fund requests from public-school teachers for everything from pencils to books; materials arrive with a disposable camera and kit for students to write notes to the donor.
Digital Wish is a registry for teachers to request tech products (cameras, computers, projectors) for their classrooms. Donors contribute funds to fulfill requests, with additional support from partnered technology companies.
Adopt-A-Classroom lets you select a classroom to contribute to and allows the teacher to choose what to buy from a list of affiliated vendors. Donors are sent a dollar-for-dollar report showing where the money went.