The Trinket has to be one of the smallest USB enabled Arduino compatible microcontrollers we've seen yet! This pretty little thing is great for when you want to keep you electronics project hidden, low-cost, and simple.

Please note: that there is only 5.25K of usable programming space (6 times smaller than the Arduino UNO) which means that this board should only be used to program simple projects. Adafruit has done an amazing job by providing a bootloader that allows USB programming but because of this, the available programming room is decreased by 2.75K. Sometimes size really does matter!   

Specifications

• Atmel ATtiny85 8-bit 8MHz Processor
• 8K (only 5.25K usable) Flash Memory
• 512 bytes RAM Memory
• 512 bytes EEPROM Memory
• 5 I/O pins - 3 analog and 3 PWM shared between the pins
• 1 I2C
• 1 SPI
• Available in both 3.3V and 5V
• On board power and indicator LEDs
• Reset button for entering bootloader or restarting the program
• Dimensions: 1.2" x 0.6" x 0.2" / 31mm x 15.5 x 5mm
• Weight (no headers): 1.85 grams