Leaning the Arduino Basics
This is a comprehensive list of links to worth reading resources on the interent that I have found during my explorations.
The official Arduino web site
It's worth reading the content of the official Arduino web siteweb site.
Here you will also find the download for the software development environment.
Don't miss the Playground that contains a lot more interesting projects and hints. If you use the search field on this site you will find all: official information, reference information and community added content.
The Blog will keep you informed.
There is also a store for buying the official boards and you can also sell your work there if you want.
Adafruit
Adafruit Industries is a company in New York that is very acive in creating new projects and making cool stuff available in their shop.
Beside selling she (Lady Ada) is very active in writing on their Blog
Wikipedia
Of course wikipedia knows about Arduino and the Arduino Plattform (in German)
Shields
A very comprehensive collection of Arduino compatible shields is availabe at
http://shieldlist.org/
Shops
... just to name a view. Be sure to contact your local shop too.
A lot of Arduino related components are also sold on eBay!
Advanced programming topics
Some tips to speed up your sketches.
More Links
The Arduino platform is also linked to the 'sister' projects: wiring, processing and fritzing
A very important source for information is the official specification for the microprocessors used with Arduino. The documentation for the ATmega328P microcontroller can be found at: http://www.atmel.com/dyn/resources/prod_documents/doc8271.pdf.
A good collection of links is available on http://www.freeduino.org/.
Nice drawings of the Arduino UNO and Mega 2560 at: http://blog.makezine.com/archive/2010/12/nice-drawings-of-the-arduino-uno-an.html
Tutorials
The famous tutorial from Limor Fried (Lady Ada) can be found at: http://www.ladyada.net/learn/arduino/index.html.
A German translation can be found at: http://www.freeduino.de/books/arduino-tutorial-lady-ada.
Creating Hardware
The EAGLE PCB Design Software from cadsoft is an easy to use, yet powerful tool for designing printed circuit boards.
A good tutorial for creating shields can be found here.
Manufacturing small amounts of shields can be done at www.jackaltac.com, www.element14.com or www.platinenbelichter.de just to name a view.
Some tips from Sparkfun at: http://www.sparkfun.com/tutorials/108.