Novakill's ANGSTkILLERmini is a freeware synth plug-in for Windows PC that's designed to be simple, yet offer surprising range.
Here's what Novakill has to say about it:
ANGSTkILLERmini is built around a simple FM oscillator with a hidden “modulator”, which uses a Sine wave to modulate the phase of the main oscillator, or “carrier”. It is very simple to use, yet capable of a surprising range of usable timbres. This is one of the best little killerz of all time!