SAYC OKbridge Simplified www.annam.co.uk/sayc.htm
Standard American Yellow Card (SAYC) by Anna Marsh and Wayne Fleury Available in Acrobat, Zip. Part 1 SAYC "OKbridge - style simplified". Part 2 Non-SAYC options which are geared to other, more complex systems, with clear explanations.

2 Over 1 "OKbridge-Style" Clarified www.annam.co.uk/2-1.html
Clarfication of 2/1 System as described on OKbridge by Anna Marsh with the help of John Hoffman (nick hoffman) Stephen (nick Stephen). Well organized, with conventions well-explained.

All of the Bridge Conventions You Could Ever Want (for now) http://home.nikocity.de/kwiese/konvent/allkonv.htm
360 conventions and treatments under 12 separate headings, as well as defenses for strong club systems. Explanations vary from barebones to full-blown, though most in between.

Blue Team Club www.mclink.itMC6246/BlueTeamClub/index.en.htm Compete system notes for the bidding system that defined the success of the Italian Open Team for over a decade.

Bridge Guys www.bridgeguys.com
Contains over 40 bidding systems including Carrot Club and Romex, and numerous conventions including Sharples and Murray Two Diamonds, as well as definitions of many bridge terms.

Hank Eng http://chemistry.ohio-state.edu/~heng/personal/b.html
Not only a host of conventions recently updated including Bourke Relays, but a description of many bidding systems, some that are not found elsewhere like Power, Leghorn Diamond and Schenken. Descriptions are variable as sources differ in quality.

Competitive Bridge www.worldwidefolks.com
While they describe 25 conventions "used in 2/1 system", many are used in other systems, including Negative Doubles and Weak Twos. Sections on competitive bidding and defensive play are also available. Teaches duplicate and tournament basics on defense and competitive bidding. Tactics section includes many topics useful for all players.

University of Warwick www.sunion.warwick.ac.uk/bridge/Acol
This seems to be a good place to start for most Acol players, though it does not address beginners specifically. The site claims to be 'reasonably complete', with topics such as: General Style; Weak Notrump; Slam Bidding; Defensive Bidding; Carding Methods.

Karen's Bridge Library www.prairienet.org/bridge
Notes avaiable for all levels, reviews, and links. Main focus is on Standard American. Wide variety of bidding and defense notes for beginners, DONT NMF & balancing in Advancing section. Intermediate articles include Negative free bids, Defenses to 2-suited overcalls, and 2C bidding structure.