0. Qucs
Download Link: http://qucs.sourceforge.net/download.html
Supported OS: Windows, Linux, Mac OSX
License: Qucs is released under the GPL license and so it is free for free programmers and users !
Qucs stands for Quite Universal Circuit Simulator. So far Qucs is not yet finished, but it is already packed with features. Take a look at the screenshots to get a feel for what it can do.
1. CEDAR
Download Link: http://sourceforge.net/projects/cedarlogic/files/latest/download
Supported OS: Windows
License: Freeware
CEDAR LS is an interactive digital logic simulator to be used for teaching of logic design or testing simple digital designs. It features both low-level logic gates as well as high-level components, including registers and a Z80 microprocessor emulater
2. Logisim
Download Link: http://sourceforge.net/projects/circuit/files/latest/download
Supported OS: Windows, Linux
License: Freeware
Logisim is an educational tool for designing and simulating digital logic circuits. With its simple toolbar interface and simulation of circuits as you build them, it is simple enough to facilitate learning the most basic concepts related to logic circuits.
3. FreeMat
Download Link: http://sourceforge.net/projects/freemat/files/latest/download?source=files
Supported OS: Windows, Linux
License: GPL
FreeMat is a free environment for rapid engineering and scientific prototyping and data processing. It is similar to commercial systems such as MATLAB from Mathworks, and IDL from Research Systems, but is Open Source. FreeMat is available under the GPL license.
4. Logic Gate Simulator
Download Link: http://sourceforge.net/projects/gatesim/files/latest/download
Supported OS: Windows
License: GPL
Logic Gate Simulator is an open-source tool for experimenting with and learning about logic gates. Features include drag-and-drop gate layout and wiring, and user created "integrated circuits".
5. Maxima
Download Link: http://sourceforge.net/projects/maxima/files/latest/download?source=recommended
Supported OS: Windows
License: GPL
Maxima is a fairly complete computer algebra system written in Common Lisp with an emphasis on symbolic computation.
6. Ngspice
Download Link: http://sourceforge.net/project/showfiles.php?group_id=38962
Supported OS: Windows, Linux
License: GPL
Ngspice is a mixed-level/mixed-signal circuit simulator. Its code is based on three open source software packages: Spice3f5, Cider1b1 and Xspice. Ngspice is part of gEDA project, a full GPL'd suite of Electronic Design Automation tools.
7. Qfsm
Download Link: http://sourceforge.net/projects/qfsm/files/latest/download
Supported OS: Windows, Linux
License: GPL
A graphical tool for designing finite state machines
8. QSapecNG
Download Link: http://sourceforge.net/projects/qsapecng/files/latest/download?source=directory
Supported OS: Windows
License: GPL
QSapecNG is a Qt-based symbolic analysis program for linear analog circuits. In fact, it consists of two indipendently parts: the SapecNG framework engine, and the application gui QSapecNG.
[source for 1-8]
