LTspice - LTspice® is a high performance SPICE simulation software, schematic capture and waveform viewer with enhancements and models for easing the simulation of analog circuits.
Proteus PCB design - Proteus PCB design combines the schematic capture and ARES PCB layout programs to provide a...
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.
KiCad - A Cross Platform and Open Source Electronics Design Automation Suite
PyLab_Works - PyLab_Works is a free and open source replacement for LabView + MatLab, written in pure Python.
OrCAD - OrCAD is a great tool for creating electronic designs that sell.