LTspice is a freely downloadable spice-based electronic simulator application produced by Linear Technology. Major features include schematic capture (circuit creation), mixed mode simulation, plus the ability to view signals using an integrated waveform viewer.
In addition to its operation as a general purpose simulator, component models are supplied for a wide range of Linear Technology devices including opamps, switched mode power supplies and transistors. Third party models may also be imported, where available.
LTspice IV is is just one of a number of applications and design tools which may be downloaded from the Software section of their website. A comprehensive range of support resources are available for LTspice from this page, including: