Multiplayer

Blitzin
The Internet Chess Club's proprietary client. Freeware, Windows only. Several languages supported.
http://www.chessclub.com/interface/download_w32.html

Checkmate
Freeware, Windows 98/ME/2000/XP. It should also run on Windows 95 and NT4 but is untested on those systems.
http://flagrantdisregard.com/checkmate/

Chess Client
Interface for FICS and ICC. Freeware, Windows 95/98 or NT.
http://www.geocities.com/TimesSquare/Alley/7007/

Chess program Pirat
Graphical interface for classical correspondence and email chess players. Includes instructions, endgame databases, engines and links.
http://www.go-pirat.de/pirat_english.htm

CompuChess
An 3D chess client for Windows.
http://www.inertiasoftware.com/chess/

Dorky Chess
A freeware Windows chess engine compatible with Winboard.
http://home.insightbb.com/~mmcknight/

Icarus
A very featureful and polished interface for ICC. Freeware, Windows only.
http://www.randomly.com/icarus/index-en.html

iNemesis
Based on CClient but has a completely new user interface and some interesting features. Works on ICC and FICS, freeware, Windows only.
http://www.ideabuilders.com/public/

LightningRulez
Based on Winboard, offers enhancements for playing speed chess on ICC and FICS. GPL licensed, Windows only.
http://www.thebugboard.net/messages/318/328.html

Net Blitz II
Playable over the Internet or against a computer.
http://www.netblitzchess.com/

SLICS
A fairly old and no longer developed interface. Not very pretty, but very fast and is quite featureful. Shareware, Windows only (including 3.1)
http://www.dfong.com/chessbd/