*Note - This list is largely based on an open source list provided by the Free Ebook Foundation - you can checkout their version of the list here.

Ada

APL

Arduino

Assembly Language

Awk

Bash

Basic

C

Chapel

Cilk

C Sharp

C++

Clojure

COBOL

CoffeeScript

ColdFusion

Coq

D

Dart

DB2

Delphi / Pascal

DTrace

Elasticsearch

Elixir

Emacs

Embedded Systems

Erlang

F Sharp

Forth

Fortran

FreeBSD

Go

Gradle

Grails

Haskell

HTML / CSS

Haxe

IoT

Isabelle/HOL

J

Java

JavaScript

Lisp

Lua

Mathematica

MATLAB

Maven

Mercurial

Oberon

Objective-C

OCaml

Octave

OpenGL ES

iPhone 3D Programming - Developing Graphical Applications with OpenGL ES - Philip Rideout

OpenSCAD

Perl

PHP

Prolog

Python

Django

Flask

The Flask Mega-Tutorial - Miguel Grinberg

R

Racket

Ruby

RSpec

Sinatra

Ruby on Rails

Rust

Sage

Scala

Scheme

Scilab

Smalltalk

Subversion

Squirrel

Squirrel Programming Course - Kelvin Shadewing

TeX

TypeScript

Unix

Git

Hadoop

Linux

Vim

Standards

Sql

MySql

NoSql

iOS

Android

D3

Mathematics

Machine Learning

Graphics Programming

Algorithms & Datastructures

Theoretical Computer Science

Operating systems

Databases

Networking

Compiler Design

Programming Paradigms

Parallel Programming

Software Architecture

Open Source

Datamining

Cellular Automata

Web Performance

MOOC

Professional Development

Security

Computer Vision

Licensing

Partial Evaluation

Partial Evaluation and Automatic Program Generation - (PDF) Jones, Gomard and Sestoft

Regular Expressions

Reverse Engineering

Misc

Other free books/reading material

Screencasts

Interactive games

Interactive/other learning material