PLASPLAS (Power Law Analysis and Simulation) is an interactive, free software program, supporting the dynamical and steady-state analysis of systems of ordinary differential equations (ode's). It was written and is maintained by Antonio Ferreira at the University of Lisbon, Portugal.
While any types of ode's can be analyzed, PLAS's primary applications are GMA and S-system models constructed with power-law functions within Biochemical Systems Theory. PLAS is closed source and does not require programming or compiling. PLAS is featured in the text Computational Analysis of Biochemical Systems. A Practical Guide for Biochemists and Molecular Biologists, xii + 530 pp., Cambridge University Press, Cambridge, U.K., 2000, by E.O. Voit.
An introductory tutorial may be downloaded here.