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.