Easiest way to become a Programmer within 30 days

Language Agnostic

Object-Oriented Reengineering Patterns

Foundations of Programming

Computer Musings

The Cathedral and the Bazaar

Patterns and Practices: Application Architecture Guide 2.0

Security Engineering

Digital Signal Processing For Engineers and Scientists

Getting Real

Domain Driven Design Quickly

Learn to Program

An Introduction to the Theory of Computation

Developers Developers Developers Developers (broken download link?)

Linkers and loaders

Let's Build a Compiler

Producing Open Source Software

How to Write Parallel Programs

Don't Just Roll the Dice

97 Things Every Programmer Should Know

How Computers Work

Introduction to Information Retrieval

Is Parallel Programming Hard, And, If So, What Can You Do About It?

Matters Computational

Type Theory and Functional Programming

Getting started with Open source development (PDF)

Database Fundamentals (PDF)

Clever Algorithms

I Am a Bug

Mining of Massive Datasets

Data-Intensive Text Processing with MapReduce (PDF)

ASP.NET MVC

NerdDinner Walkthrough

Assembly Language

ProgrammingGroundUp (PDF)

Paul Carter's Tutorial on x86 Assembly

Software optimization resources by Agner Fog

C / C++

The new C standard - an annotated reference

Matters Computational: Ideas, Algorithms, Source Code, by Jorg Arndt

The C book

Thinking in C++, Second Edition

C++ Annotations

Software optimization resources by Agner Fog

Introduction to Design Patterns in C++ with Qt 4 (PDF)

Object Oriented Programming in C (PDF)

Beej's Guide to Network Programming

Learn C the hard way

The Definitive C++ Book Guide and List

C#

C# School (covers C# 1.0 and 2.0)

Threading in C#

C# Yellow Book (intro to programming)

C# Programming - Wikibook

C# Essentials

Data Structures and Algorithms with Object-Oriented Design Patterns in C#

Illustrated C# 2008 (.zip, dead link)

Delphi / Pascal

Essential Pascal Version 1 and 2

The Tomes of Delphi

HTML / CSS

Dive Into HTML5

HTML Dog Tutorials

Java

Sun's Java Tutorials

Thinking in Java

How to Think Like a Computer Scientist

Java Thin-Client Programming

OSGi in Practice (PDF)

Java 6 Tutorial (PDF)

JavaScript

Eloquent JavaScript

Crockford's JavaScript

jQuery Fundamentals 

Mozilla Developer Network's JavaScript Guide

Essential Javascript & jQuery Design Patterns for Beginners

Up and Running with Node

The Node Beginner Book

Mastering Node.js

Linux

Advanced Linux Programming

.NET

C# School 

Visual Studio Tips and Tricks 

Entity Framework

Charles Petzold's .NET Book 0

Threading in C#

C# Yellow Book (intro to programming)

C# Programming - Wikibook

C# Essentials

Data Structures and Algorithms with Object-Oriented Design Patterns in C#

Nemerle

Oracle Server

Oracle's Guides and Manuals

Oracle PL/SQL

PL/SQL Language Reference

PL/SQL Packages and Types Reference

Steven Feuerstein's PL/SQL Obsession - Videos and Presentations

Parrot / Perl 6

Using Perl 6 

Perl

Higher-Order Perl

Perl The Hard Way

Extreme Perl

Perl Free Online EBooks (meta-list)

The Mason Book

Practical mod_perl

Beginning Perl

Embedding Perl in HTML with Mason

Perl & LWP

Perl for the Web

Web Client Programming with Perl

Modern Perl 5

PHP

Symfony2

Practical PHP Programming 

Zend Framework: Survive the Deep End

Python

Byte of Python

Building Skills in Python Version 2.5

Python Bibliotheca

Think Python (PDF)

Data Structures and Algorithms in Python

Dive into Python

How to Think Like a Computer Scientist: Learning with Python

Python for Fun

Invent Your Own Computer Games With Python

Learn Python The Hard Way

Thinking in Python

The Django Book

Snake Wrangling For Kids

Natural Language Processing with Python

Ruby

Programming Ruby

Why's (Poignant) Guide to Ruby (mirror)

Mr. Neighborly's Humble Little Ruby Book

Ruby Best Practices

MacRuby: The Definitive Guide

Learn Ruby the hard way

Ruby on Rails

Ruby on Rails Tutorial: Learn Rails By Example

Scala

Programming in Scala, First Edition

A Scala Tutorial for Java programmers (PDF)

Scala By Example (PDF)

Programming Scala

Xtrace (Github)

Lift (Github)

Pro Scala: Monadic Design Patterns for the Web

Exploring Lift 

SQL

Developing Time-Oriented Database Applications in SQL

Use The Index, Luke! 

Learn SQL The Hard Way