Built around the Atmel ATtiny85 a little chip with a lot of power. The Attiny85 is a fun processor because despite being so small it has 8K of flash and 5 I/O pins including analog inputs and PWM 'analog' outputs. Has a USB bootloader so you can plug it into any computer and reprogram it over a USB port just like an Arduino.
Adding simple modifications to the Arduino IDE the Trinket works like a miniArduino board. You can't stack a big shield on it but for many small & simple projects the Trinket will be your goto platform.
This is the 3V Trinket with a MicroB USB jack for power and/or USB uploading. With 8k of flash 2.75K is taken by the bootloader leaving approximately ~5.25K bytes available for use. Interface with sensors and devices that need 3V logic or when you want to power it off of a LiPo battery. The 3V version should only run at 8 MHz. Can be overclocked to 16 MHz but your programs might not run correctly.
Figure 1: Trinket Dimensions