Some of the most useful sites that I have come to know are as follows. Anyone who has spent more than a couple of months would definitely be knowing these websites. The following is meant for new comers as well as others to make recommendations:
Money Saving SitesMoney Saving Expert - Comparisons between products/ services and various money saving tips
Money Super Market - Same as above but much more famous; less advice and more comparison
Quidco - Gives you charge back on online purchases; if you are buying online, don't forget to get a charge back!
Price Runner - Price comparison portal
Trading SitesGixen - eBay Auction Sniper
Auto Trader - Everything cars!
Abe Books - for used books
Accommodation and Settling InRight Move - Find a place to rent
Argos - Buy it, argos it!
Tech Startups, News and Misc.Techcrunch Europe - Techcrunch targeted at European news
Do It - Volunteers and opportunities
The list doesn't include some travel related sites like WY Metro and London Journey Planner, which are location specific. It doesn't include sites like Groupon which I think have extremely appalling offers in the UK.