Tools that use ngspice as simulator

In more than 10 years of maintenance and development, ngspice earned a good reputation among free spice-based simulators.  Ngspice is used as simulator in different tools, both free and commercial. Here is the list of the tools we know about (in alphabetical order). You may find graphical user interfaces with or without schematic capture, circuit development tools as well as development environments for models and devices

Tools for circuit simulation with ngspice
Blackboard allows to generate circuits on stripboards and uses ngspice for circuit simulation.
ngspice integration into the CADENCE circuit design environment is promoted by a toolset offered here.
CoolSpice is a SPICE suite and circuit simulation software package. CoolSpice contains ngspice, a schematics editor with GUI, a plotter application, and a text editor for manually editing netlists.
Cppsim is a system simulator that integrates ngspice as transistor-level simulator. Ngspice shares the schematic capture and postprocessing tools with the other simulators included in the suite (C++ and Verilog).
The gEDA project is a full suite and toolkit of Electronic Design Automation tools. It uses ngspice for circuit simulation.
KiCad is a cross platform and open source electronics design automation suite. Its schematic editor Eeschema fully integrates shared ngspice as the simulation tool. Whereas this feature is not yet part of the actual KiCad release, the code is already available in the master branch, and also compiled as a nightly build for MS Windows. A Youtube video gives a short introduction.
Kjwaves is a frontend for ngspice written in Java using NetBeans and runs on any platform where you can install 1.5 release of Java virtual machine. Kjwaves can be used as a simple plotting program to display the content of a spice RAW file or as a complete frontend to the simulator. Information on using Kjwaves with ngspice is found here. Unfortunately its development has stalled since 2007.
Layouteditor includes a schematic editor with an interface to ngspice for circuit simulation.
MSEspice is a handy graphical front-end for ngspice. It is made with the MSEide+MSEgui cross platform development environment for Free Pascal.
NGSPICE online is available in your web browser, again using ngspice as back-end engine. The application allows entering netlists and commands from the local browser window. The simulation is run on a remote server and the results are transferred back to the web browser for visualization and analysis.

eSim is an open source EDA tool for circuit design, simulation, analysis and PCB design. It is an integrated tool built using open source software packages KiCad, ngspice and Scilab and runs on LINUX and MS Windows.
PartSim is available in your web browser, again using ngspice as back-end engine. The application allows creating circuits with schematic entry from within the local browser window. The simulation is run on a remote server and the results are transferred back to the web browser for visualization and analysis.
Volta is a GUI to ngspice, including schematic capture. It is specially designed for the MAC OS X operating system.
WeSpice is an Android application for circuit simulation that actually runs ngspice simulation engine on tablets. WeSpice allows to build hierarchical schematics, symbols and models, perform simulations and analyze results, locally on the table without permanent Internet connection.
XCircuit is an open source UNIX/X11 (and Windows) program for drawing publishable-quality electrical circuit schematic diagrams and related figures, and produce ngspice circuit netlists through schematic capture.

Tools for system and device modeling (and others) integrating ngspice
FEKO is a comprehensive electromagnetic simulation software tool, based on state of the art computational electromagnetics (CEM) techniques. It enables users to solve a wide range of electromagnetic problems. ngspice allows the integration of SPICE subcircuits for non-radiating networks as an addition to the FEKO electromagnetic models.
GNU Octave is a high-level language, primarily intended for numerical computations. It provides a convenient command line interface for solving linear and nonlinear problems numerically, and for performing other numerical experiments using a language that is mostly compatible with Matlab. Information on interfacing Octave with ngspice is found here. Octave may be used for post-processing (e.g. plotting) ngspice rawfile data.
i-MOS is an open platform for model developers and circuit designers to interact. Model developers can implement their models over the i-MOS platform to promote their acceptance and obtain user feedback. Circuit designers can use the platform to try out the most recent models of many newly developed devices. The platform provides ngspice as an online simulation engine.
RandomSpice is a Monte Carlo circuit simulation engine, which enables large-scale simulations of statistical and process variability, data harvesting and statistical analysis of the simulation results. It supports using multiple instances of ngspice.
rlabplus for LINUX is an interactive, interpreted scientific programming environment which provides fast prototyping and program development, similar to Matlab(c). It integrates ngspice as a simulation tool.