Free Circuit Simulators

LTspice

It is a circuit simulator, SPICE simulator, schematic capture and waveform viewer. It is a very useful software for analog circuit simulation and also very easy to use. This will help you in transient, ac, noise and dc analysis as well as Fourier analysis. It is extensively used by various educational institution and students. 
Note: Download the software from the following download link. If it does not work then go to the website link and then manually go to the download page.
Download LTspice      Website

SimOne

This includes capture schematic editor which helps in schematic reading and editing, component selection, and simulation control of analog circuits. The features include DC analysis, AC analysis, Transient analysis, PSS analysis, Temperature sweep, Parameter sweep, Stability analysis.

Note: Download the software from the following download link. If it does not work then go to the website link and then manually go to the download page.
Download SimOne      Website

SapWin

This is a very useful tool which will help you to create the schematic of a linear analog circuit, to find network function in Laplace domain and to show the results in graphic form. In this tool you will get all passive and active, bipolar and two-port, linear components, including RLC elements, operational amplifiers and small-signal equivalent models of BJT and MOSFET transistors. This tool is available at University of Florence website.
Note: Download the software from the following download link. If it does not work then go to the website link and then manually go to the download page.
Download SapWin         Website

Micro-Cap

This is useful for both analog and digital circuit simulations and also it is SPICE compatible. This has been developed by Spectrum Software. It was first released as Micro-Cap package in 1982. Since that day it has been steadily expanded and improved. This is the first logic simulation system to be available in personal computers. The latest version is Micro-Cap 10.
Note: Download the software from the following download link. If it does not work then go to the website link and then manually go to the download page.
Download Micro-Cap     Website

Logisim

It is a digital circuit simulator having a graphical user interface similar to traditional drawing. One of its interesting feature is that it allows simulation of circuits as you build them. It has been written completely in JAVA programming language using Swing. It allows to build the circuits from truth tables using combinational analysis module.
Note: Download the software from the following download link. If it does not work then go to the website link and then manually go to the download page.
Download Logisim        Website 

5Spice

With this tool you can design spice models and sub-circuits easily with a GUI schematic editor. You can define, name and save unlimited numbers of analyses, each with a corresponding graph. It also has support for analysis of alternative designs. After installation on the right side of the screen you will get a toll bar from where you can drag and drop all the components like resistor, capacitor, diode, op-amp, logic gates, voltage source etc.

Note: Download the software from the following download link. If it does not work then go to the website link and then manually go to the download page.

Download 5Spice     Website