Ninecows has released The Putney v2.0, a freeware architectural replica (SynthEdit) of the old Putney by EMS, also known as VCS3 or Synthi A.

From the manual (included in the download):

As the original it is constructed of basic modules connected in a matrix, letting you route sources and control voltages to different destinations. Many module based synths can do that, but the Putney has one remarkable difference: There is no clear distinction between audio and control voltages as sources. For instance; you can let the output of one audio oscillator control the frequency of another, or use the noise generator to control the filters cut-off frequency.
Even though the synth (just as the original) is monophonic, the flexible routing system will let you create a huge variety of different sounds and effects.

It actually looks like a Portabella (Synthi A) instead of the Putney.

The Putney v2.0 features

• Oscilators - Osc 1: Triangle and Sinus shaped wave, Osc 2 & Osc 3: Triangle and Square shaped wave (Osc 3 operates in an approximately 20 times lower frequency range, useful for LFO).
• Noise generator - produces white noise, which is passed through a filter letting you change the colour of the noise with the colour knob.
• Filter/Oscillator - signals assigned to the Filter column of the matrix will be summed and send to the filter.
• Envelope generator - Attack, On, Decay and Off.
• Trapezoid - control voltage produced by the envelope generator is duplicated and send to the trapezoid module where it can be modified further by adjusting its level or adding/subtracting a DC voltage.
• Effects - Ring modulator, Reverberation (spring reverb), Delay.
• Output channels - VU meters and tuner which enables you to tune the oscillators to the standard 440 Hz tuning.
• The Matrix - Route sources and desitination (also inverting signals).

Note: the plug-in is a VST effect and not a VST instrument, despite its capability to be midi-controlled.

Download The Putney v2.0 here.