www.lastminute.com - before booking ANY theatre I make sure they don't have a special deal here - often including dinner for cheaper than a ticket cost.
www.uswitch.com -find a better deal on broadband, gas, electricity - pretty much anything!
www.buymobilephones.net - I have talked about this before - switch to a better mobile phone deal or get a free gift if you stick to a similar contract (by gifts I mean laptops, digital cameras, ipads etc).
Trivago - great for hotels. I have problems with Bookings.com (who offer cheap rates but in fact, you just get the worst room in the Hotel)
Let me know in the comments box of any that you like, in particular - I would love to find out more...