An advanced network simulator to design and configure virtual networks
Build, Design and Test your network in a risk-free virtual environment and access the largest networking community to help. Whether you are studying for your first networking exam or building out a state-wide telecommunications network, GNS3 offers an easy way to design and build networks of any size without the need for hardware.
gpsim is an open sourced simulator for Microchip's PIC microcontrollers. It supports all three families of PICs: 12-bit, 14-bit, and 16-bit cores. See also gputils http://gputils.sourceforge.net/
This is a very simple simulator for the Microchip PIC16C71. I've used it for various small projects. It has an option video display feature to simulate a software-generated video output, which I used to debug PIC-Pong.