Name me some VSTi synths that emulate a specific piece of hardware.  IOW, not samplers that have libraries but things like OrganizedTrio, Mr. Tramp and that sort of thing.

Eventide "Blackhole"...it actually only emulates one algorithym of an Eventide hardware processor...but a dang good algorithym:-)

There are a lot of Solina emulations such as Cheese Machine or Crazy Diamonds (as in Shine On ...) The ultimate one trick pony might be the one that emulates the Tibetan monks.

There are the MusicLab series of specific guitar emulators, RealGuitar, RealLpc, and RealStrat.
They will be releasing version 3 very soon also.

G-Force Mellotron (pro).

yorolpal


G-Force Mellotron (pro).

And their Minimonsta (mini moog), Oddity (ARP Oddesy) and Imposcar (OSC OSCAR). However seeing these are emulations of very versitile synths maybe they don't count as a one trick pony? 
 
One of my favorite hardware emulations is Korg's legacy Mono/Poly.
 
 

Arturia

http://www.arturia.com/evolution/

Mod Bod

...things like OrganizedTrio, Mr. Tramp and that sort of thing.

You've mentioned two of GSi's products.  I presume that you have checked out their others, e.g. Miles'tone, MrRay and VB3.

http://www.genuinesoundware.com/?a=products

DSK seems to specialize in one-trick-pony softsynths.

http://www.dskmusic.com/category/vsti-all/

These are all free.

Here are two classic Italian transistor organs that nail their respective emulations:

Martinic Combo Model F (Farfisa)
Martinic Combo Model V (Vox Continental)

And they are free downloads!