People have been thinking about a photon-photon collider (see this and this) as an add-on or to supplement the ILC, the next generation linear collider.
My question is, have there been any photon-photon colliders running in the past*, including those running below the $e^+ e^-$ threshold to study non-linear effects in photon scattering?
*Virtual/Weissäcker-Williams/Equivalent photon-photon colliders don't count.