Official and wikipedia
Shields & Communication
Tutorials
Arduino clones
- Standuino – unite the Ardunio witth a prototyping board and you get the Standunio
Paperduino
Inputs
Outputs
Miscellaneous
CNC
Robotics
Misc
See also Suppliers#Arduinos and Shields on where to get Arduinos, shields, sensors and related stuff and Interfaces#I/O Boards for a list of alternative interface boards.