<p>Where can I find a free, very quick, and reliable implementation of FFT in C#?</p>

<p>That can be used in a product? Or are there any restrictions?</p>

AForge.net
is a free (open-source) library with Fast Fourier Transform support. (See Sources/Imaging/
ComplexImage.cs
for usage, Sources/Math/
FourierTransform.cs
for implemenation)