Arduino-Tiny is an open source set of ATtiny "cores" for the Arduino platform.

The Arduino platform currently supports Atmel ATmega processors. There is a need for the Arduino platform to work with physically smaller DIP package processors. The intent of this project is fulfill that need. Specifically, our goal is to provide a core that enables Arduino users to work with the ATtiny84 (84/44/24), ATtiny85 (85/45/25), and ATtiny2313 (4313) processors.

Download the latest version for Arduino 1.0

Download the latest version for Arduino 1.5

Arduino libraries available for ATtiny processors...

I2C / TWI Master library for the ATtiny85

Alternatives to this core (as of 2010-Nov-16)...

bohne (René Bohne) published '2313 and '84 cores... http://www.luminet.cc/

SuperCow (R.Wiersma) published an '84 core... http://85.12.32.240/~superkoe/arduino/attiny84-03.zip

saposoft (Alessandro Saporetti) published a '45 core... http://sites.google.com/site/elettronicaarduinoesperimenti/Home/arduino-ide-and-attiny45

The MIT High-Low Tech group published a '45 / '85 core... http://hlt.media.mit.edu/?p=1695

I believe macsimski has published an '85 core but I can't find a link.

Sofian Audry and Samuel St-Aubin published a '13 core... http://accrochages.drone.ws/en/node/38

Mark S (Mark Sproul) has a multi-processor core that supports several ATtinys... http://www.avr-developers.com/

(Thank you to everyone above!)

(Apologies to anyone missing from the list. Anyone not included is simply a lack of awareness on my part.)