They are all simulators for electronic circuits, just like LTspice or TINA-TI, but different.
1.
QUCS (Quite Universal Circuit Simulator) - is the one usually found in Linux repositories, and the simulation engine is based on Qucsator, it is not based on SPICE
2.
Qucs-S (Qucs with Spice) - has the look of QUCS, but the simulation engine is based on SPICE, more precise 'ngspice'. Usually not in the Linux repositories, has to be compiled or installed manually. Besides Qucs-S, ngspice must also be installed.
3.
QucsStudio (IMO this is the best Qucs) - has the look of QUCS, it is released as a portable version for Windows only, but it works just fine in Linux with Wine and it is all built with GNU components, but compiled for Windows/MinGW only. Just unzip and run bin/qucs.exe with WINE on Linux, or run the start.bat for Windows.
QucsStudio has the best GUI, a good set of video tutorials, and what I like most, it has live Sliders (called Tune), look how awesome this feature is (at minute 3:55)!
- All Qucs versions have some nice RF goodies e.g. microstrip simulation, filters design, Smith charts, etc.
- For non RF simulations, LTspice is easier to operate and have more parts models when in comparison with Qucs or TINA-TI.
- Components models and libraries seems different between Spice and Qucsator engines.
So far my pick is
QucsStudio for Qucs like simulators and
LTspice for Spice like simulators.
Are there other FREE simulators I should try?