Here's an excerpt from the About SCOAP3 page:
To address this situation for HEP and, as an experiment, Science at large, a new model for OA publishing has emerged: SCOAP3 (Sponsoring Consortium for Open Access Publishing in Particle Physics). In this model, HEP funding agencies and libraries, which today purchase journal subscriptions to implicitly support the peer-review service, federate to explicitly cover its cost, while publishers make the electronic versions of their journals free to read. Authors are not directly charged to publish their articles OA. . . .
Each SCOAP3 partner will finance its contribution by canceling journal subscriptions. Each country will contribute according to its share of HEP publishing. The transition to OA will be facilitated by the fact that the large majority of HEP articles are published in just six peer-reviewed journals. Of course, the SCOAP3 model is open to any, present or future, high-quality HEP journal aiming at a dynamic market with healthy competition and broader choice.