Let me start with MindView Inc – Bruce wrote, may be, the best books you can find on Java and C++ and they are free on the web.

Meta-List

• How to Design Programs: An Introduction to Computing and Programming
• 25 Free Computer Science Ebooks
• Free Tech Books
• Wikibooks: Programming
• Cheat Sheets (Free)
• CodePlex List of Free E-Books
• Book Training – On Video!
• Sofware Program Managers Network – Free EBooks
• EBook Share @ linbai.info
• FreeBooksClub.Net
• Theassayer.org
• O’Reilly’s Open Books Project
• TechBooksForFree.com

Language Agnostic:

• Foundations of Programming by Karl Seguin
• Computer Musings (Lectures by Donald Knuth)
• The Cathedral and the Bazaar (Introduction to Open Source Software)
• Patterns and Practices: Application Architecture Guide 2.0
• Security Engineering
• Digital Signal Processing For Engineers and Scientists
• Getting Real (Courtesy 37 Signals)
• Structure and Interpretation of Computer Programs
• Domain Driven Design Quickly
• OO Design
• Best Kept Secrets of Peer Code Review
• NASA Software Measurement Handbook
• NASA Manager Handbook for Software Development
• Introduction to Functional Programming – Class Lectures and Slides
• How to Design Programs – MIT Press
• Guide to the Software Engineering Body of Knowledge – IEEE Computer Society Press
• Online Course Materials – MIT
• Data Structures and Algorithms
• Essential Skills for Agile Development
• Programming Languages: Application and Interpretation
• Learn to Program
• Patterns of Software: Tales from the Software Community
• How to write Unmaintainable Code
• The Art of Unix Programming
• The Definitive Guide to Building Code Quality
• How to Think Like a Computer Scientist
• Planning Algorithms
• The Little Book of Semaphores
• Mathematical Logic – an Introduction
• An Introduction to the Theory of Computation
• Developers Developers Developers Developers
• Linkers and loaders
• Beej’s Guide to Network Programming
• Domain Driven Design Quickly
• 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 – Cambridge University Press. 2008.

Android:

• The Complete Android Guide

Bash

• Advanced Bash-Scripting Guide
• Lhunath’s Bash Guide

C/C++

• The new C standard – an annotated reference
• 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 (Open Publication License) (PDF)

Django

• Djangobook.com

Git

• Pro Git
• The Git Community Book

HTML

• Dive Into HTML5

Java

• Sun’s Java Tutorials
• Thinking in Java
• How to Think Like a Computer Scientist
• Java Thin-Client Programming
• OSGi in Practice (CreativeCommons Attribution Non-commercial Share Alike License)

JavaScript

• Eloquent JavaScript
• Crockford’s JavaScript
• jQuery Fundamentals (starts with JavaScript basics)

Linux

• Advanced Linux Programming

Maven

• Better Builds with Maven
• Maven by Example
• Maven: The Definitive Guide

Mercurial

• Mercurial: The Definitive Guide
• HGInit – Mercurial Tutorial by Joel Spolsky

NoSQL

• CouchDB: The Definitive Guide

Objective-C

• The Objective-C Programming Language

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 (Open Publication License)
• Perl & LWP
• Perl for the Web
• Web Client Programming with Perl
• Modern Perl 5

PHP

• Practical PHP Programming (Wiki that contains O’Reilly’s PHP In a Nutshell)
• Zend Framework: Survive the Deep End

PostgreSQL

• Practical PostgreSQL

Python

• Dive Into Python
• Dive Into Python 3
• Byte of Python
• Building Skills in Python Version 2.5
• Python Free Online Ebooks Meta-List
• Python Bibliotheca
• Think Python
• Data Structures and Algorithms in 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 (nltk-book)

Ruby

• Programming Ruby
• Why’s (Poignant) Guide to Ruby (Mirror via Scribd)
• Mr. Neighborly’s Humble Little Ruby Book
• Ruby Best Practices
• MacRuby: The Definitive Guide
• Ruby on Rails Tutorial: Learn Rails By Example

Scala

• A Scala Tutorial for Java programmers
• Scala By Example
• Programming Scala
• Xtrace (Github)
• List (Github)
• Pro Scala: Monadic Design Patterns for the Web
• Exploring Lift (published earlier as “The Definitive Guide to Lift”, pdf)

Subversion

• Subversion Version Control: Using the Subversion Version Control System in Development Projects
• Version Control with Subversion

SQL (Implementation agnostic)

• Developing Time-Oriented Database Applications in SQL,Richard T. Snodgrass
• Use The Index, Luke! – A guide to SQL database performance for developers