An Italian guy (Fabio Varesano) has designed and built the smaller Arduino clone available:
It has almost the size of a Bluetooth module. Really great work and the most importantly, Fabio shares all essential resources so that one can build Femtoduino. The schematics are based on the Arduino Pro Mini by Sparkfun.
Now, Internet/Web of Things is much closer to hobbyists with projects that can be more pervasive and easily deployed!
Check the video link below to see also it’s size against an Arduino Uno board:
More information and links to schematics and instructions here.
**You can pre-order one here!**