ESynth is a free program designed to explain the harmonic analysis and synthesis of signals. With ESynth you can create signals by adding together individual sinusoidal waveforms (sinewaves) and study the resulting waveform and spectrum. You can also perform an analysis of an input waveform, to see how a given sound can be represented in terms of a sum of sinewaves. More information.
ESYSTEM is a free program for experimenting with signals and systems. With ESYSTEM you can see the effect of simple systems on a range of simple signals. More information.