Some interesting Arduino resources.
General
Arduino official homepage
Wikipedia
The biggest list of shields
Blogs
Makezine
Manicbug
Tronixstuff
Wicked device blog
Elecfreaks
Bildr
HifiDuino
Arduino Passion
Slice Of Raspberry Pi
Projects
Freeduino : A long list of projects and examples
Makezine
Stream internet radio
Build a wireless sensor network
Instructables
Hack A Day
Control tv/radio/blue-ray players… with your iPhone
Projects on YouTube
Software repository github
Guides and tutorials
Ladyada arduino beginners guide
Hacknmod
Wireless tutorial
Multi-protocol infrared library
Other
Just the bare components
Arduino internals
Tutorials on USB, SPI, I2C, …