The source code for these plugins can be found in the VapourSynth source tree. Many of them are included in the Windows installer in DLL form.
AVISource()
AVIFileSource()
OpenDMLSource()
eedi3()
Write()
Read()
AverageFrames()
Hysteresis()
SCDetect()
Dilate()
Erode()
Open()
Close()
TopHat()
BottomHat()
Recognize()
RemoveGrain()
Repair()
Clense()
ForwardClense()
BackwardClense()
VerticalCleaner()
TextFile()
Subtitle()
ImageFile()
Vinverse()
VFM()
VDecimate()