SparkFun's minimal design approach to Arduino. This is a 5V Arduino running the 16MHz bootloader (select 'Arduino Duemilanove w/ 328' within the Arduino software).
This is a 3.3V Arduino running the 8MHz bootloader, in a very small package - a Mini Arduino, you might say!
You need to wedge an Arduino into a really tiny spot? Simple - just use a SMD '328 IC. Oh... you want a reset, and voltage regulator too? Why didn't you say so!
This is LilyPad Arduino, the heart of a wearable electronics system! This main board features a ATmega168V preloaded with Arduino, and is ready to connect to a wide variety of wearable, sewable modules!
The Arduino Leonardo is a microcontroller board based on the ATmega32u4 (in Documentation tab).
The Pro Micro is similar to the Arduino Pro Mini except with an ATmega32U4 on board. The USB transceiver inside the 32U4 allows us to add USB connectivity on-board and do away with bulky external USB interface.
The Netduino 2 is an 32-bit open source electronics platform using the .NET Micro Framework with a rich development environment.
Netduino Plus 2 expands on the impressive 32-bit open source Netduino by adding ethernet connectivity and an SD card socket.