Csound is an extremely powerful program, not only to generate samples but you can even create entire pieces of music with it, apply effects to existing sounds, do MIDI in/out, VST, LADSPA, and others. This is a program I have effectively done many things with.
However, to make some simple synthesis I have the internal synthesizer in AmigaMML, which can generate square waves, saw waves, triangle waves, sine waves, noise, FM synthesis, and combinations of such.