An application note (PDF!) from Microchip on using the Ultra Low-power Wake-up module to reduce system current consumption:
This application note describes ways to reduce system current consumption with the use of the Ultra Low-power Wake-up (ULPWU) module. The PIC16F684 and PIC16F88X are examples of deviceswith this feature.
The primary use of this module is as an ULPWU timer, but its functionality can be expanded to function as a temperature sensor and/or a low-voltage detector. The main and expanded functions of this module are explained in this document.