Orangator by T-Rex

This is an interesting little soft synth which is actually surprisingly capable for its time. It has an insane amount of possible oscillators, some pretty decent effects, and pretty good filters too. It can be played realtime via MIDI, or sequenced via the built-in tracker-style sequencer. There are a number of ways to generate and shape waveforms, including drawing your own. For 1997 this was a pretty awesome tool (especially for free)- I still actually use this synth sometimes to generate waves for samplers, or just odd stuff to mangle for sound design purposes.

Later there was a modular version called Visual Orangator, but I don't think it ever really got finished, and personally I always preferred this one.