Price comparison sites

I couldn't see a thread for price searching sites so here's what I've found so far:

http://www.buydigital.tv

http://www.idealo.co.uk

http://www.twenga.co.uk

http://www.pricerunner.co.uk

Anyone have any others? TV's are pretty easy to find but other kit is more difficult. I also find some advertise at a lower price but do not stock the item. I wonder if this is a marketing ploy?