Two B-factories were designed and built in the 1990s, and they operated from late 1999 onward: the Belle experiment at the KEKB collider in Tsukuba, Japan, and the BaBar experiment at the PEP-II collider at SLAC in California, United States.
They were both electron-positron colliders with the center of mass energy tuned to the ϒ(4S) resonance peak, which is just above the threshold for decay into two B mesons (both experiments took smaller data samples at different center of mass energies).
BaBar prematurely ceased data collection in 2008 due to budget cuts, but Belle ran until 2010, when it stopped data collection both because it had reached its intended integrated luminosity and because construction was to begin on upgrades to the experiment (see below).
However, Belle II successfully started taking data in 2018 and is currently the only next-generation B-factory in operation.
[6] In addition to Belle II there is the LHCb-experiment at the LHC (CERN), which started operations in 2010 and studies primarily the physics of bottom-quark containing hadrons, and thus could be understood to be a B-factory of this "next generation."