The proper name is either a 6+2 pin or 6 pin PCIe power cable, depending on how many pins it has. You're looking for something like this!

The reason for the confusing name is that it is typically used for powering graphics cards that plug into your PCIe slot.

EDIT: These cables can only carry so much current through them before they melt or catch on fire (150w for a 6+2, 75w for a 6). BE CAREFUL when using splitters to power your miners. It's not always a bad idea (eg when powering a heavily undervolted antminer S1), but it frequently can create a fire hazard. Remember to always have a PSU that exceeds your power requirement, you don't want to have consistent load over 80%.