Gpsim is a full-featured software simulator for Microchip
PIC microcontrollers distributed under the GNU General
Public License, Version 2 or higher, and some of it's libraries
under GNU Lesser General Public License, Version 2 or higher.

Gpsim has been designed to be as accurate as possible.
Accuracy includes the entire PIC -- from the core to the I/O pins
and including ALL of the internal peripherals. Thus it's possible
to create stimuli and tie them to the I/O pins and test the PIC
the same PIC the same way you would in the real world.