The official Arduino environment supports ATmega8, ATmega168, ATmega328P, ATmega1280, and ATmega2560.
Minor tweaks get you a number of other basically-compatible processors (ATmega88, ATmega328, ATmega32, some others.) See
http://www.avr-developers.com/More significant tweaks get you ATmega644 ("Sanguino") and ATmega1284.
Also assorted ATtiny cpus
http://code.google.com/p/arduino-tiny/"Wiring" is arduino-like and includes some other AVRs:
http://wiring.org.co/"Energia" supports a similar environment for MSP430.
Note that the Arduino IDE is currently in transition from a version where adding new CPUs is "relatively difficult" to a version (1.5.x) where it is "relatively easier."
"Supports" is somewhat ambiguous; many published arduino projects use MCU capabilities beyond those that are part of the Arduino environment itself, using capabilities that are specific to a particular board/chip. Those will require additional work to get running on other CPUs; it may not even be possible.