tkCybernetics is a small grafical tool for rapidly setting up simulations of simple cybernetic systems. It was designed for didactic purpose but might be useful for the quick test of some circuit too.
The Graphical Display and Analysis Tool (GDAT) is a flexible and extensible data graphing program suitable for engineering applications. It can display and measure waveforms (e.g., rise time), and it can be extended through a plugin architecture.