Winboard and Xboard

Beowulf
WinBoard and XBoard chess engine that runs on Windows, UNIX and Linux. Source code is available.
http://www.frayn.net/beowulf/

Delfi
A freeware engine.
http://www.msbsoftware.it/delfi/

Faile
A free engine runs with WinBoard and Xboard. The source code is also available under the GNU GPL software license.
http://faile.sourceforge.net/

Gargamella
An Italian engine. Includes downloads.
http://digilander.libero.it/gargamellachess/

Golem
An engine.
http://www.geocities.com/TimesSquare/Chaos/9481/

Green Light Chess
Free program that plays chess, either with its own graphical interface, or using Tim Mann's Winboard program. Estimated ELO 2300.
http://www.7sun.com/chess/

Jim Ablett's Winboard Projects
Contains links of Winboard projects like MSCP chess engine, java executables, Pgn-Extract-XP.
http://homepages.tesco.net/henry.ablett/jims.html

Kiwi
Winboard and XBoard chess engine with opening book.
http://www.ascotti.org/programming/chess/chess.htm

Thor's Hammer
Winboard chess engine in C++. Executable and source code available.
http://www.geocities.com/toma_st/thorshammer.html

A Tribute to Winboard
An excellent page from a Winboard fan, that has everything from A to Z about Winboard.
http://www.edcollins.com/chess/winboard.htm

Xadreco
XBoard, WinBoard and UCI compatible engine. Includes source code and e-groups.
http://www.geocities.com/pag_sax/xadreco/index-en.html

XBoard and WinBoard
A graphic interface program. Includes news, FAQs, and downloads.
http://www.tim-mann.org/xboard.html

Xpdnt
Chess engine for Winboard/Xboard. Has basic opening book and adjustable hash size.
http://free-chess.home.comcast.net/