
In quasi-historical order:

Original Authors:
Davy Temperley: http://theory.esm.rochester.edu/temperley/
Daniel Sleator: http://www.cs.cmu.edu/~sleator/
John Lafferty: http://www.cs.cmu.edu/~lafferty/

Much additional work has been done by many different developers.
The current lead developer is
Linas Vepstas: http://linas.org/

Arabic dictionaries were provided by
Warren Casbeer and Jon Dehdari

Farsi dictionaries were provided by
Jon Dehdari

The SAT parser code was developed by
Filip Maric <filipmaric_at_gmail.com>
as a part of the 2008 Google Summer of Code (GSoC) project for OpenCog.

Russian dictionaries were provided by
Sergey Protasov <svp_at_tj.ru>

Support for Hebrew, a complete overhaul of tokenization, and assorted
general maintenance and performance work:
Amir Plivatsky <amirpli_at_gmail.com>

Turkish dictionaries by:
Can John Bruce <can.bruce_at_yale.edu>
Turkish sharply revised:
Tatiana Batura, Maria Mitkovskaya, Natalya Semenova

Indonesian dictionaries were provided by:
Hendy Irawan (ceefour)

Kazakh dictionaries from:
Tatiana Batura, Aigerim Bakiyeva, Aigerim Yerimbetova

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Some of the people who have contributed patches and fixes include:
(In alphabetical order)

Artem Gayardo-Matrosov
Bill Hayes <bhayes84_at_gmail.com>
Borislav Iordanov <borislav.iordanov_at_gmail.com>
Valery Kholodkov <valery_at_grid.net.ru>
Vikas N Kumar <walburn_at_gmail.com>
Dom Lachowicz <domlachowicz_at_gmail.com>
Per Larsson <tucker_at_algonet.se>
Stephen Leibowitz <librestephen_at_gmail.com>
Marc Maurer <uwog_at_uwog.net>
Evgenii Philippov <efilippov_at_gmail.com>
Sergey Protasov <svp_at_tj.ru>
Sampo Pyysalo <smp_at_is.s.u-tokyo.ac.jp>
René Rhéaume <rene.rheaume_at_gmail.com>
Lian Ruiting <lianlian1022_at_gmail.com>
Murilo Saraiva de Queiroz <muriloq_at_gmail.com>
Fridrich Strba <fridrich.strba_at_bluewin.ch>
Simon Stuart <kraven_at_greycascade.com>
Peter Szolovits <psz_at_mit.edu>
Dimitri Timofeev <dtim@kalami.org>
Alexander Tkachuk <alexander.tkachuk_at_gmail.com>
Vincent Untz <vuntz_at_opensuse.org>
Max Vyaznikov http://myclerk.ru
Bruce Wilcox <gowilcox_at_gmail.com>
