Segor (Electronics shop in Berlin)
http://www.segor.de
Reichelt (internet electronics shop)
http://www.reichelt.de/
Tinker Soup (Berlin based sparkfun reseller, Arduino)
http://tinkersoup.de/
Watterott (another Germany based sparkfun and adaFruit reseller, Arduino)
http://www.watterott.com/
Sparkfun (US based DIY electronics shop, Arduino)
http://sparkfun.com/
AdaFruit (US based DIY electronics shop)
http://www.adafruit.com/