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.
Arduino libraries available for ATtiny processors...
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://184.108.40.206/~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.)