FlightGear Flight Simulator: free open-source multiplatform flight sim
IEC 101 Source code Library Windows Linux POSIX - c, c++, c# .net
The classic RPL programming language reimplemented and reimagined.
Simulate your IO with this Arduino Simulator
Modbus slave simulator
RetroPC-SIM is an Arduino project to help to understand how pc works
Open source model airplane simulator
Circuit simulators like SPICE for low memory computers
A simulator for Atmel's 8/16-bit AVR microcontrollers
Multithreaded N-Point Gravity Simulation for PC and Android
Various TCP/IP servers, clients and network management tools
SDK for the SARK-110 Antenna Analyzer
To be a practical Arduino simulator