KiCad - A Cross Platform and Open Source Electronics Design Automation Suite
FMOD - FMOD Studio is an audio middleware solution and engine for games.
Proteus PCB design - Proteus PCB design combines the schematic capture and ARES PCB layout programs to provide a...
Wwise - Game audio engine, designed to give artists more control and save programmers' time.
QUCS - Qucs, briefly for Quite Universal Circuit Simulator, is an integrated circuit simulator which means you are able to setup a circuit with a graphical user interface (GUI) and simulate the large-signal, small-signal and noise behaviour of the circuit.
SoLoud - Easy to use, free, portable c/c++ audio engine for games.