Dota 2 Tools:
Dota Pro Game Statistics: Datdota.com
Dota 2 Hero Calculator
A great way to think up new item builds for heroes and see how they affect the hero's stats and damage.
Dota 2 Statistics & Patch Comparisons
Drafting help (picks/bans), hero winrates and pick rates between patches, etc.
Dota 2 Build Creator & Rating
Similar to the above, although a bit less detailed, but allows people to rate your build.
Dota 2 Drafting Tool by Winrate
An interesting graphical representation of a draft, based on hero win-rate and hero strengths. The weakness of this tool is that it does not compare two opposing drafts.
I find this tool works best by picking a carry first, and then seeing what heroes it recommends from there.
Dota 2 Drafting Tool and Combo Creator
A slightly different take on a drafting helper. Also allows you to create and/or rate others' hero combos.
Dota 2 Hero Stats by JankDota
A sortable list by almost every hero stat available.
Dota 2 Draft Site
A great low-pressure way to practice your drafting. It's all theorycraft, though - there's no way to test your draft afterwards (other than getting critiques from other players). You can see some results of my drafting practice and analysis.
Dota 2 Interactive Map
Shows / measures effect and vision radius, neutral camp spawn boxes, etc. Neat tool to play around with.
Dota 2 Team Finder
Find yourself an appropriate Dota 2 team! (sign-in via Steam)
Dota 2 Stat Parser: YASP
Dota 2 Guides:
My favourite site for builds and guides. Sando and Peppo's guides are especially good.
Purge's Guide to Dota2: "Welcome, You Suck."
A great guide for beginners. Doesn't beat around the bush, gives you the basics.
Beginner's Guide to Dota on Reddit
Some things that beginners may not think about. Very well written, focuses on game-play and less on mechanics and heroes.
Slashstrike's Guide to Solo Ranked
An interesting take on why "attitude" loses you games. Some perspectives on here I hadn't considered in quite that light.
ChaQ's Guide to Advanced Solo Mid
This guide is probably the best I've ever seen, written by a near-pro who found out just what he was missing, and tells of all the little pro-level things you need to know to beat an opponent mid.
Tsunami's Hero Tips
(somewhat outdated, but a fun read).
BurningSera's Guide to Supporting
One of the better support guides I've found, with the right "tone".
Comprehensive Support Guide by Ler on Liquiddota:
This guide is probably the most "strategic" support guide I've ever read. It's a little more abstract, but covers important concepts for any player intending to head towards higher-level play.
Official e-book release (post)
Guide to Picking a Support Hero
This guide is behind current meta of 6.82b (at the time of this posting), but its thought process is good on how to evaluate a support.
DotaFire Guide to Picking a Support Hero
I like this guide mainly for the categories of supports.
Guide to Level 1 Roshan Drafting
Teamliquid post - well thought out with lots of examples. You can also see some of my lists related to heroes, including List of Roshan Heroes by Lane, List of Roshan Heroes by Function, and some more Level 1 Roshan Team Ideas.
Guide to Better Dota by W.
How to Tri-Lane by Peppo
Understanding Power Levels by Skim
A support's view on what timings can be used to take advantage of your power vs. enemy's power at any point in the game.
Duncan Geere's How to learn a hero in two hours
Quickly learn the basics about any hero with this simple method!
Rudd's Draft Unpredictably - How Versatility Helps You Win
A very useful guide for learning which heroes are versatile.
NinjaMovesPro's How to be a Pro
An all-round, really well written, guide to becoming a better DOTA player. This guide is heavily based on it.
SkyStormSpectre's TP Scroll - What to know
A ridiculously detailed guide to TP scrolls. It's a must read.
Hamstertamer's Countering Common Pubstomp Heroes
A newer guide with useful tips on how to beat those "OP heroes".
Sando's Drafting and Team Strategy
Become a master of DOTA tactics with this guide to proper hero drafting!
BurningSera's How To Play Support
A complete (and funny) guide to playing supports. Very detailed.
Chris Thursten's Three Lane Highway: How to Communicate Effectively in Solo Ranked Matchmaking.
All the articles of his Three Lane Highway series make excellent reads, especially this one.
Chris Thursten's Three Lane Highway: Heroes of Solo Ranked Matchmaking
Here's a new Three Lane Highway article I really enjoyed!
Gaijindash's Dota 2: Mindset and You
A very in-depth look at how your state of mind affects your MMR. Almost too much detail.
Chris Thursten's Why playing the blame game will always hold you back
Dedicated to anyone who tells me they're not to blame for being in the trench.
Avoiding Basic Errors - A Noob's Guide