Solve Elec - Solve Elec is a free educational program to draw and analyze electrical circuits.
LabVIEW - LabVIEW offers many of the functions you'd find in a traditional script-based programming language presented in a more intuitive visual interface ideal fo engineering.
simavr - a lean and mean Atmel AVR simulator for linux
itom - itom is an open source software suite for operating measurement systems, laboratory automation and...
Emulino - an emulator for the Arduino platform by Greg Hewgill
PyLab_Works - PyLab_Works is a free and open source replacement for LabView + MatLab, written in pure Python.