Ray Wu Connector or 13.5mm
Probably the most prevalent connector out there. This one has been around for a while. You can tell you have a Ray Wu style connector by verifying that the nut is on the male side, and the notch in the middle is square.
Paul Zhang Connector or 18.5mm
Another popular connector that is compatible with CFOL style connectors. This is very much like the Ray Wu connector but bigger. It also has the nut on the male side, however, the notch is rounded instead of a square.
The xConnect connector is compatible with Scott LED and HC brand ends. This connector is different in that the nut is on the female side while keeping a square notch in the center. This is around the same size as a Ray Wu style connector.