Below is a list of recommended threads on a variety of topics. We suggest you read through these to avoid opening a similar thread. Also remember that only the best of the best are posted in this list - there is a completely separate wealth of information that can be found using the search function as well.
Credit to GroT and Taiche for the original Recommended Threads.
Recommended threads now have a '!' before their title. Do not start a thread and put the ! in yourself, I will edit it in when I link it to the recommended threads.
GENERAL GUIDES
Beginner
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Definitions - Defines most terms you may be unfamiliar with.
Day[9]'s Podcasts - Talks about a variety of subjects. Everyone should listen.
Strategy vs tactics, or "why did I lose?" - A great guide, with numerous examples, about balancing strategy and tactics in your play.
Stay One Step Ahead - Walks through all the fundamentals of the mental side of StarCraft.
Basic Advice to People Asking for Help - Outlines the four things you should always be focusing on during a game of Starcraft.
Ctrl and Shift Usage - How to use these buttons to simplify certain situations.
Advanced
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Magic Boxes: Keys to Spreading Spells - A hidden box in Starcraft determines how units work.
Timing - Descriptions of the various types of timing.
Costs of Cancelling - When you get charged and when it is free.
Adding Production Facilities - How stopping unit production to make production facilities can put you ahead.
Splash Damage - Explains specifically how splash damage works.
Dark Swarm - Explains how Dark Swarms works with various examples.
Ensnare Effects - How ensnare affects units' attack rates differently.
RACE SPECIFIC GUIDES
Protoss
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General Protoss
Reaver Dropping - How to get the most out of this drop.
Protoss Hotkeys - Protoss Hotkey Tips
PvP
PvP Guide - BigBalls explains his main PvP builds.
PvT
PvT Guide
A Beginner Push-Breaking Guide - A lengthy guide on breaking Terran's Tank push.
Counter Terran Turtle - How to stop a turtling Terran's Vulture harassment into big 1/1 push.
Killing Mines with Dark Templar
When to get Observer speed - Definitive timing on when it should be researched.
PvZ
PvZ Basics - Good overview of how to play this matchup.
4 Gate 2 Archon - An indepth look at this powerful build.
PvZ Strategy Guide - Strong funamental guide with good replies.
Understand the Matchup - FrozenArbiter makes several corrections.
Forge FE - Where to place your buildings when you fast expand on popular maps.
2 Gate - Fairly recent discussion of the viability of 2 Gate, with good replies.
The Logic of Larvae - Discussion on killing Zerg units based on their larva cost.
Terran
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General Terran
Walling in - How to build the ideal wall on nearly every popular map.
Terran FAQ - Covers almost everything about Terran. The first place you should start.
Stylish VODs - A huge detailed explanation of Terran strategy. Contains build orders, and links to commentated VODs and replays for every build. Every Terran player should watch these to learn.
DaZe's Advice for Terrans - DaZe covers all three Terran matchups.
TvP
Early Game TvP - Exhaustive guide on early game TvP.
Early Game TvP - A little outdated, but still applicable.
MnM + Tank Timing Rush - Recent guide with explanation of the pros and cons of this rush.
Building Placement - How certain building placements fare against Zealots.
TvT
TvZ
Adapting after FE - How to change your build after fast expanding.
The Art of 3 Tank 1 Vessel - A VERY detailed look at this common, strong build.
Timing - How to time your attacks
Mid-game Tips
SK Terran - Guide to playing without Tanks.
Defending Mutalisks
3 Barracks Build - Sunken-breaking build.
The logic of larvae - Discussion on killing Zerg units based on their larva cost.
Zerg
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General Zerg
Complete Zerg Guide
Zerg Mechanics - Some tips on improving Zerg control.
iNc's Tips for Zerg - iNcontroL covers all three Zerg matchups.
iNc's Modern Zerg - iNcontroL updates all three Zerg matchups.
12 Pool Zerg Guide - Good guide covering the less conventional 12 Pool.
Mutalisk Guide - An explination of all things involving Mutalisks.
How to move Larvae - How you can force Larvae to move to the left of a Hatchery.
Getting a queen every game? - Discussion of when Queens are useful and which abilities to use.
ZvP
5 Hatch Hydra into Muta Build - A very common, modern build.
The Art of ZvP - Explanation of modern ZvP concepts.
Up-to-date ZvP Guide
ZvP basics: how to abuse the crackling - Originally written for Luna, but the information still applies.
ZvT
ZvT on Luna - Originally written for Luna, but the information still applies.
3 Hatch Mutas - A guide on this build order.
3 Hatch Mutas Early Game - Fairly advanced guide discussing many overlooked aspects.
Stopping the Blob - Gudie on stopping Terran's midgame pushes.
Stopping the Bunker Rush
Beating Mech
ZvZ
12 Hatch vs 12 Pool - The advantages and disadvantages of this matchup.
Mutalisks vs Scourge - The two methods for reliably killing Scourge with Mutalisks.
Beating Scourge/Expo off one base
Beating HydraLurker - Basic advice on beating a Hydralisk user.
TEAM GUIDES
2v2
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2on2 For Dummies (Youtube Channel) - Commentated FPVODs of very high level 2v2 play. A great resource for 2v2 players.
Playing Zerg in 2v2 - A random assortment of builds and strategies.
Playing PT - Brief advice on each of the matchups.
Playing TZ - Plenty of good advice on playing this team in 2v2.
MAP SPECIFIC GUIDES
Current Competitive Maps
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Old or Casual Maps
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ZvT on Neo Forte
ZvT on R-Point
PvZ on Rush Hour - FrozenArbiter's guide on his style - "Sim City Toss"
Blood Bath Guide - Huge thread on playing Blood Bath.
Blood Bath Blog - Summary of how to play Blood Bath.
OBSOLETE GUIDES
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BigBalls on PvZ - Originally written for Lost Temple, but the fundamentals still hold up.
BigBalls on PvT - Also written for Lost Temple. Otherwise still applicable.
Stopping 9/10 Gate - Somewhat obsolete because they encourage Hatch at ramp, but still contains good information.