HummingBoard: Will fit in a Raspberry Pi enclosure, runs Linux or Android, packs a bigger punch. Caution: While the pins are similar to Raspberry Pi, installing Pi hardware takes extra effort (not a direct replacement).
Shrunk down an Arduino to the size of a finger-tip! by Open Source RF, via Kickstarter. RFduino: A finger-tip sized, Arduino compatible, wireless enabled microcontroller, low cost enough to leave in all of your projects!