general vendors of electronic components:
http://www.conrad.nl
http://www.farnellinone.nl
http://www.reichelt.de
http://www.rs-components.com
http://www.jameco.com (USA based)
http://www.nedis.nl
http://www.jelu.se (lots of cool stuff, like nokia display drivers)
http://www.octopart.com (datasheets and price info)
offline stores
http://www.baco-army-goods.nl
http://radio-rotor.nl/
http://www.muco-modelbouw.nl/
http://www.muco.nl/
http://www.hecke.nl/
vendors of specific electronic components:
http://www.microchip.com
http://sample.microchip.com
van Oijen Technische Informatica (his webshop looks a bit clumpsy and unprofessional -especially in comparison with the others-, but this guy is friendly and his goods delivers in good shape and time. For if you are particulary fond of doing business with dutch natives)
http://www.melabs.com
http://www.adafruit.com
http://www.sparkfun.com (suggested by johannes)
http://www.sparkfun.com/commerce/distributors.php#eur (sparkfun distr. europe) http://www.eztronics.nl (dutch version like sparkfun)
http://www.olimex.com (suggested by johannes especially for printing circuits)
http://www.modtronix.com (pic boards with ethernet and embedded webserver)
http://www.hebeiltd.com.cn/?p=z.pricelist.led.diode (leds in all colors)
http://www.sonmicro.com (for all your rfid readers and stuff)
http://www.antratek.nl (lots of products from sparkfun/olimex/parallax etc)
http://www.pololu.com (good for robotics)
http://www.probotics.nl/Shop/index.php (more robotics)
http://www.stronglink.cn (cheap rfid reader modules)
http://shop.jeelabs.com/ (dutch arduino clone vendor)
http://nl.rs-online.com/web/ (various arduinos and [proto/xbee/ethernet], free next day delivery)
http://www.textilkabel.de/ Nice textile covered cable.
http://www.pieterfloris.nl/shop/ Dutch reseller for arduino stuff and more
http://www.watterott.com (German reseller of sparkfun/adafruit/makerbot and much more, low cost and speedy shipping)
http://www.opitec.nl (cheap DC motors, gears, solar panels and more)
PCB manufacturing
http://www.batchpcb.com/
http://www.eurocircuits.com/