LTspice comes bundled with a full suite for circuit design: schematic editor, simulator, and waveform viewer. It is actively developed, reasonably fast and very easy to use.
ngspice has a lot more features, including some scripting, and optional integration with TCL (tclspice), or even GPU processing (CUSPICE). ngspice is also Free (as in freedom) software licensed under the modified BSD license.
Xyce is developed at Sandia National Laboratories. It’s main feature is support for parallel multi-core simulation. Xyce is also Free software licensed under GPL v3.