WOK has released RING-O, a free VST effect plug-in for Windows for ring modulation effects.
RING-O can use the left and right part of a stereo input as sources for the modulation, or an input is ring-modulated by the internal oscillator. Different waveforms, MIDI or manual pitch control and continuous mixing offer plenty of sound mangling options.
In addition the plugin features a clean "math" form of ring modulation and a more dirty "diode" emulation.
Price: Free.