BoomPlug has released BoomComb, a free tuneable comb filter VST2.4 plugin.
Here's what BoomPlug has to say about it:
BoomComb is a delay based effect where it feeds back a delayed version of itself.
BoomComb can be used as like a flange like effect or can be used like a resonator with the frequency controlled by midi. It can also be used to create string like sounds via inputting noise in it. Another use is it can be used to add space to a sound and can be useful for spicing up synthetic drum sounds to make them sound a little more interesting.