Tools that use ngspice as simulator

In more than 15 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. The author has stopped its development, the sources are available here.
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).
Eagle is the well known and widely distributed PCB layout software for every engineer. It offers a complete set of PCB layout and schematic editing tools, library content, and community-driven features. The recent version 8.4 integrates ngspice for circuit simulation.

EasyEDA is a tool for online circuit simulation, PCB design, and Electronic circuit design. ngspice is the simulation engine.
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.
The gEDA project is a full suite and toolkit of Electronic Design Automation tools. It uses ngspice for circuit simulation.
Isotel has developed an open source software kit for mixed signal simulation and synthesis using Eagle and ngspice. A combination of mature open source packages yields a usable and working solution of Verilog simulation with mixed-signal spice, integrating ngspice with Yosys for Verilog RTL synthesis, Icarus Verilog for Verilog simulation and synthesis, gtkwave for debugging on Verilog simulation models, and graphviz for graph visualization.
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.

oregano - an electrical engineering tool
Oregano is an application for schematic capture and simulation of electronic circuits. The actual simulation is performed by ngspice or other simulators.
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.
Qucs-S (Qucs with Spice) is a Qucs subproject using free SPICE circuit simulation kernels (ngspice recommended) with the Qucs GUI. It merges the power of SPICE and the simplicity of the Qucs GUI.
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
BBSPICE with its online simulator is a set of Python scripts to enable simulation for synthetic biology with ngspice. A description is found here.
efabless has been set up to re-invent the semiconductor product creation. The company applies collective and multidisciplinary community knowledge to all aspects of semiconductor product development. It uses ngspice in the tool chain.
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.
MATLAB may be used for post-processing of ngspice simulation results. A script is availble for loading the simulation output (ngspice rawfile) into MATLAB.
PSIM is a design and simulation software to empower engineers in the power electronics design industry. PSIM's SPICE Module, powered by CoolCAD Electronics' CoolSpice, and thus using ngspice as its engine, provides the capability to run SPICE simulation in the PSIM environment.
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.