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 mini-Arduino board. You can't stack a big shield on it but for many small & simple projects the Trinket will be your go-to platform.

This is the 3V Trinket with a Micro-B 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.