Scilab is a free and open source software for engineers & scientists, with a long history (first release in 1994) and a growing community .
MATLAB & Simulink
MATLAB is a multi-paradigm numerical computing environment and proprietary programming language developed by MathWorks. MATLAB allows matrix manipulations, plotting of functions and data, implementation of algorithms, creation of user interfaces, and interfacing with programs written in other languages.
MATPOWER is a package of free, open-source Matlab-language M-files for solving steady-state power system simulation and optimization problems, such as power flow (PF), continuation power flow (CPF), extensible optimal power flow (OPF), etc.
PowerWorld Simulator is an interactive power system simulation package designed to simulate high voltage power system operation on a time frame ranging from several minutes to several days. The software contains a highly effective power flow analysis package capable of efficiently solving systems of up to 250,000 buses