gpsim is a full-featured software simulator for Microchip PIC microcontrollers
distributed under the GNU General Public License.

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.

gpsim has been designed to be as fast as possible. Real time simulation speeds
of 20Mhz pics are possible. A 'goto $' program runs at the same speed as a
25Mhz pic when simulated on my 400Mhz PII Linux Box. Of course, as you add
stimuli and begin interacting with peripherals, the performance drops. But
it's still fast!

gpsim has been designed to be as useful as possible (at least that's the
intent - honest). The standard simulation paradigm including breakpoints,
single stepping, disassembling, memory inspect & change, and so on has been
implemented. In addition, gpsim supports many debugging features that are only
available with in-circuit emulators. For example, a continuous trace buffer
tracks every action of the simulator (whether you want it or not). Also, it's
possible to set read and write break points on values (e.g. break if a
specific value is read from or written to a register).

WWW: http://www.dattalo.com/gnupic/gpsim.html