Ngspice is a mixed-level/mixed-signal circuit simulator. Its code is based on three open source software packages: Spice3f5, Cider1b1 and Xspice. Ngspice is part of gEDA project, a full GPL'd suite of Electronic Design Automation tools.
Spice3 does not need any introduction, is the most popular circuit simulator. In over 30 years of its life Spice3 has become a de-facto standard for simulating circuits.
Cider couples Spice3f5 circuit level simulator to DSIM device simulator to provide greater simulation accuracy of critical devices. DSIM devices are described in terms of their structures and materials.
Xspice is an extension to Spice3C1 that provides code modeling support and simulation of digital components through an embedded event driven algorithm.
Ngspice is, anyway, a little more than the simple sum of the packages above, as many people contributed to the project with their experience, their bug fixes and their improvements. If you are interested, browse the site and discover what ngspice offers and what needs. If you think you can help, join the development team.
Ngspice is an ongoing project, growing everyday from users contributions, suggestions and reports. What we will be able to do depends mostly on user interests, contributions and feedback.