USB : Diecimila
“The Arduino Diecimila is a microcontroller board based on the ATmega168 (datasheet). It has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16 MHz crystal oscillator, a USB connection, a power jack, an ICSP header, and a reset button. It contains everything needed to support the microcontroller; simply connect it to a computer with a USB cable or power it with a AC-to-DC adapter or battery to get started. ” (http://www.arduino.cc/en/Main/ArduinoBoardDiecimila) I like this board a lot since it has both 5V and 3.3V out that is very useful when you are testing things with . It can be used as quick voltage regulator. Also, you can take off the chip and use it as USB-Serial converter or even use it as bootloader programmer. (Here is a useful link that shows how to burn bootloader using arduino>>http://www.geocities.jp/arduino_diecimila/bootloader/index_en.html)
“The Arduino Mini is a small microcontroller board based on the ATmega168 (datasheet), intended for use on breadboards and when space is at a premium. It has 14 digital input/output pins (of which 6 can be used as PWM outputs), 8 analog inputs, and a 16 MHz crystal oscillator. It can be programmed with the Mini USB adapter or other USB or RS232 to TTL serial adapter.” (http://www.arduino.cc/en/Main/ArduinoBoardMini)
“Arduino Nano is a surface mount breadboard embedded version with integrated USB. It is a smallest, complete, and breadboard friendly. The Nano was designed and is being produced by Gravitech.
It has everything that Diecimila has (electrically) with more analog input pins and onboard +5V AREF jumper. Physically, it is missing power jack and power select jumper. Since the Nano is automatically sense and switch to the higher potential source of power, there is no need for the power select jumper. ” (http://www.arduino.cc/en/Main/ArduinoBoardNano)
“The LilyPad Arduino is a microcontroller board designed for wearables and e-textiles. It can be sewn to fabric and similarly mounted power supplies, sensors and actuators with . The board is based on the ATmega168V (the low-power version of the ATmega168) (datasheet) or the ATmega328V (datasheet). The LilyPad Arduino was designed and developed by Leah Buechley and SparkFun Electronics.” (http://www.arduino.cc/en/Main/ArduinoBoardLilyPad)
There are nice comparison on various Arduino on Arduino.cc site.