Arduino Booklet - written by Massimo Banzi - has become the O'Reilly book "Getting Started With Arduino" (Must be purchased) here
Arduino Comic - created by Jody Culkin - this is a very simple introduction to Arduino that will help you out during your first days. It is a Creative Commons document, please check out if there is a translation to your language and if not, feel free to make one and push it back the site
Tutorial - Learn electronics using Arduino: covers the fundamental things you can do with Arduino. Very detailed, with lots of pictures and videos.
Tutorials - Regular, supported Arduino tutorial series: From blinking an LED to complex timing, wireless communication including XBee and GSM cellular, GPS and display systems - detailed tutorials with worked examples, sketches, photos and videos.
freeduino.begin() - written by Siddharth Sharangpani & Bhagyashri Sharangpani, is an ultimate beginners guide to Arduino/Freeduino platform. Complete DIY approach with indepth coverage of various aspects of the platform ranging from Digital input/output, Analog Input/Output, Memory, Serial communication, Infra red, XBEE, Bluetooth, Make your own robot, .NET Framework C# integration with freeduino and Interrupts. Detailed journey into the world of physical computing. Free Sample chapter. Available for purchase on iTunes store for iPad and PC/Mac editions. For USD $2.99 only.
freeduino.begin(), The ultimate beginners guide to Freeduino/Arduino platform
Arduino in a Nutshell - written by Professor Jan Borchers - is a quick, hands-on, basic introduction to Arduino. Unlike many other books, it's kept very up to date - the latest edition 1.6 is from January 2013 and covers the current Arduino Uno R3 board and version 1.0.3 of the Arduino IDE. It's ideal for those who know a little programming but no electronics. The PDF is available for free here: Arduino in a Nutshell booklet.
Arduino Programming Notebook - written by Brian Evans in its first edition can be
purchased as a 40 page paperback at publishing cost at LULU, or
downloaded for free as a pdf: arduino_notebook_v1-1.pdf
City University of Hong Kong, School of Creative Media - runs undergraduate / graduate level courses on the Arduino & basic electronics for artists: Arduino & basic electronics for artists class website
Arduino How-To WIKI - Detailed How-To for connecting many Input and Output devices to Arduino: Starting with Arduino, Arduino Power!: Relays and DC power control, Environmental Sensors, Ultrasonic rangers, Wireless link, and more.
Getting Started - Designers guide to getting started with electronic sketching
CourseWare - This CourseWare page aims to create "course materials that can be used at different educational institutions."
The Complete Beginners Guide to the Arduino - EarthShineElectronics - Beginners guide to using the Arduino. Contains tutorial projects.
Quarkstream - Arduino tutorials for high school Physics class.
Sheepdog's Guide to Arduino Programming - About 35,000 words spread over 17 chapters. Most useful if you read the chapters in sequence. Some essential hardware issues explained along the way, but mostly about the programming. You can start it as a novice. You should leave it not a novice.
Blinkenlight Experiments - Experiments with very simple hardware - The experiments are related to each other and exhibit a lot of concepts beyond just blinking. Difficulty levels range from beginner to expert.Suitable for classroom because of the simple hardware. Most experiments will only require 20 LEDs. A dedicated shield is available but not mandatory.
Arduino Education - High School Curriculum Curriculum for high school courses that use Arduino to teach basics of electronics, programming, and microcontrollers. Both course are still under development. Physical Computing is a virtual class that will run for the first time in the summer of 2010. Electronics is still pretty rough but will be seeing continual development. 