Shopping
Computers
Ebuyer. Good site layout means it's easy to find things. Quite cheap too.
Price comparison
Pricerunner. Books, films, computers, utilities, finance etc.. Compare prices across hundreds of online retailers
Kelkoo do a similar sort of thing but it's often possible to buy cheaper than these sites claim.
Clothes
Prototype 21. Fantastic Japanese style on their Terratag label.
Cyberdog. Cool clubbing clobber. Flashing T-shirts etc.