Wikipedia List of Arduino Compatibles
http://en.wikipedia.org/wiki/List_of_Arduino_compatibles

Arduino Uno
http://arduino.cc/en/Main/ArduinoBoardUno

Menta
http://blog.makezine.com/2012/05/01/whats-different-about-the-mintronics-menta/

Seeduino
http://www.seeedstudio.com/blog/tag/seeeduino/

Ruggeduino
http://ruggedcircuits.com/html/ruggeduino.html

Arduino Pro
http://arduino.cc/it/Main/ArduinoBoardPro

Arduino Nano
http://arduino.cc/en/Main/ArduinoBoardNano

Solarbotics Ardweeny
http://www.solarbotics.com/product/kardw/

Boarduino
http://www.ladyada.net/make/boarduino/

Teensy
http://www.pjrc.com/teensy/

Sparkfun Pro Micro AT32U4
http://www.sparkfun.com/tutorials/338
http://www.sparkfun.com/products/11098

Adafruit AT32U4
http://www.ladyada.net/products/atmega32u4breakout/
http://www.adafruit.com/products/296

JEE Node
http://shop.moderndevice.com/products/jeenode-kit

Maple Leaf
http://leaflabs.com/devices/maple/

Arduino Due and Leonardo
http://arduino.cc/blog/2011/09/17/arduino-launches-new-products-in-maker-faire/

Hamstack
http://www.hamstack.com/hamstack.html

Raspberry PI
http://www.raspberrypi.org/

Beaglebone
http://beagleboard.org/bone

Itead Studio Breadboard Power
http://iteadstudio.com/store/index.php?main_page=product_info&cPath=18&products_id=438

Arduino Stacking Headers and Jumpers
http://www.adafruit.com/products/85
https://www.adafruit.com/products/266
http://www.sparkfun.com/products/10007
http://evilmadscience.com/productsmenu/partsmenu/251
Or your favorite auction site for bulk orders.

Sparkfun FTDI Basic Breakout - 3.3V logic
http://www.sparkfun.com/products/10009

Sparkfun FTDI Cable - 3.3V logic
http://www.sparkfun.com/products/9717

Solarbotics TTLy FTDI
http://www.solarbotics.com/product/39240/

USB Bub 2
http://shop.moderndevice.com/products/bub_ii

Sparkfun Proto Shield
http://www.sparkfun.com/products/7914

Freetronics Prototyping Shield
http://www.freetronics.com/products/protoshield-basic

Evil Mad Science Googly-Eye Shield
http://www.evilmadscientist.com/article.php/googlyshield

Wicked Devices Ardweeny Prototyping Shield
http://shield.wickeddevice.com/