I own a Arduino UNO with the ATmega328 Microcontroller.
Which other Microcontrollers are compatible to the Arduino system?
There are several varieties of Arduino available. All are based on the Atmel AVR 8 bit microcontrollers.
The UNO has a DIP socket which can support the ATMega8, ATMega168 or the ATMega328. See this thread on the Arduino forum for more detail.
Generally Arduino (if not all) use Atmel AVR 8-bit MC's. Which IMO is a good thing since I find them personally easier to use (and they are usually cheaper too).
Not that PICS are bad or anything...