ATtiny is a family of small, low-power 8-bit RISC microcontrollers from Atmel. Commonly found in smaller Arduino prototyping platform boards and projects.
They are often used for hobby projects, since many tools are available for free.
A popular use of ATtiny microcontrollers is the arduino prototyping platform system which has bootloaders and software targeting the small mcus, and V-USB, which is a software implementation of the USB Slave Peripheral protocol.
The bigger brother of the ATtiny is the atmega.