Bridge is it
Concerned with bridge bidding systems, especially CLONE, based on a forcing weak/strong 1 club opening. Also features links to other bridge resources, particularly on bidding systems. Bilingual site (English/Norwegian).
http://home.no.net/raptor/

An Unassuming Club
A bidding system invented by Donald A. Varvel of Mountain View, CA, USA. It is based on an ambiguous 1 club opening, associated with 5-card majors, weak no trumps, and weak 2s.
http://www.blakjak.demon.co.uk/auc_don.htm

Conventions
Survey of a large number of commonly used bridge bidding conventions and bidding systems.
http://www.bridgeguys.com/conventions.html

The Blue Team Club System
Description of a complex bidding system invented in Italy. A major part of the information is in downloadable files only. Bilingual site (Italian/English).
http://www.mclink.it/personal/MC6246/BlueTeamClub/sistema.htm

The L System
An original bridge bidding system presented by Hong Liu, based on very light openings.
http://www.angelfire.com/wy/hl/

M.A.F. - The Ultimate Bridge Bidding System
A scientific bidding system from the Netherlands (5-card majors and different two-suiter openings). Bilingual site gives information in English and Dutch.
http://web.inter.nl.net/hcc/M.A.F/

Precision Bidding System
Detailed description of the Precision Bidding System (strong 1 club opening, with 16+ points), as modified by one partnership.
http://acp.atlanta.ga.us/wang/bridge/gdum111.htm

Contract Bridge Culbertson System
Contract bridge bidding, as developed by Culbertson. Culbertson's survey of good playing techniques is also included.
http://www.stjost.si/bridge/

Bridge Bidding Guide
Look-up tables for bridge bidding, with the most common conventions used in "Standard American" bidding, by Richard Pavlicek.
http://www.rpbridge.net/bgtc.htm

Brazilian Bridge
Presentation of the Carioca Club and other conventions.
http://sites.uol.com.br/webridge/english/

Bidding Quest
Practice bidding on prepared deals with scores. Compare and check how other pairs have bid same boards. Try bidding competition with anonymous partner.
http://www.biddingquest.com/

The Diamond Major
Forcing club, 5-card major, medium notrump system.
http://www.bridgeclublive.com/Include/Diamond.htm

Bridge Conventions
Provides an encyclopaedia of conventions, systems and treatments.
http://www.gwilliams.org.uk/

Neill, Daniel
Proposes bidding notes to download for many systems, conventions and treatments.
http://www.geocities.com/daniel_neill_2000/sys/

An Unassuming Club
Presentation of a variant of the Polish Club system.
http://www.bridge.hotmail.ru/file/Red/auc/club.html

Precision Two-over-One
Presents an overview of a system mixing a strong club with standard methods.
http://www.p21.150m.com/

Bridge World Standard
Description of a bridge bidding system based on the preferences of leading players, and aimed at the needs of occasional partnerships. Some characteristics: 5-card major opening, strong no-trump, artificial strong two-club opening, weak 2s in the other suits, and inverted minors.
http://www.bridgeworld.com/default.asp?d=bw_standard&f=bwstandardmainpage.html

Bidding by Design
Proposes articles on bidding system theory and offers examples for download. Free system design software available for download.
http://www.xs4all.nl/~jisbert/

B-M Precision
Detailed presentation of a variation of the system.
http://members.aol.com/evboyer/bmp/

Aggressive Bidding Foundation
Proposes an approach based on a math model over computer analysis. Provides famous pairs systems, an exposition of the theory and a bidding machine.
http://public.aci.on.ca/~zpetkov/

Polish Standard 2000
Description of the Polish Club bidding system, with different variants.
http://www.wj2000.front.ru/

Bidding Order Shows Strength (BOSS)
Describes a system where priority is given to the strength of the partnership hands.
http://www.jhardie.pwp.blueyonder.co.uk/boss.htm

REGRESsion
Modification of the bidding system Regres, based on frequent openings with weak hands and opening passes with strong hands.
http://members.ams.chello.nl/a.denbroeder4/index.html

The Chilli Bidding System
Gives priority to simplicity and universality of rules.
http://chillibidding.org/

Caliper
Bidding system from Norway based on a Big Diamond relay sequence.
http://home.no.net/foevsted/

Bridge Guys
Lists and defines a large number of systems and conventions.
http://www.bridgeguys.com/

Bridge Files
Provides overview of different systems, a list of conventions and information about the game. Free registration required.
http://www.bridgefiles.net/

Colonial Acol and LISA
Presentation of two bidding systems, one a US adaptation of the British Acol system, the other a modification of Standard American.
http://www.kentfeiler.com/bridge/home/bridge.htm

Coyot's 2/1 Game Force
Detailed presentation of this american system.
http://web.pinknet.cz/~coyot/sa21gf/sa21gf_en.php

Inquiry 2/1 GF
Online book prensenting the author's variation of this popular north american system.
http://inquiry2over1.blogspot.com/

Zar Points
Aggressive hand evaluation and bidding based on distribution, controls, and hcp. Presents the Zar Bid Machine.
http://www.zarpoints.com

Bridge Conventions
A thorough guide to the various bridge bidding and playing conventions used around the world.
http://homepage.mac.com/bridgeguys//conventions.html

Bridge Conventions
Presents an list of explanations and definitions.
http://www.pattayabridge.com/convindex_main.htm

SA-YC Simplified
This bidding system is a simplified version (as played on OKBridge) of the Standard Yellow Card (SA-YC) system of the ACBL (American Contract Bridge League). Its essential features include the strong no-trump, 5-card major and weak 2 openings.
http://www.fifthchair.org/archive/annam/sayc.htm