Leviton 37A00-1 Lighting Controls Auxiliary Switch, White
The Auxiliary Switch can be used with any HAI by Leviton Lighting Control (HLC) switch to make "three-way" (where two switches control one light) and multi-way (where more than two switches control one light) switches without using conventional "three-way" wiring techniques.  HLC products work as stand-alone or with a Home Control System.