Hotels.com - easy to use site with listing for hotels all over the world. Ongoing promotion of 1 free night for every 10 nights booked.
Lastminute.com - huge range of hotels globally and Top Secret Hotels which you can book blind for big discounts.
iescape - beautiful range of handpicked accommodation all over the world with really helpful descriptions. Each listing has 'highs' and 'lows' and there is lots of information about child friendly options.
Websites which have a great range of hotels to book as well as flights, car hire and a whole host of other things:
Price comparison Websites which let you compare the cost of the same room across lots of different accommodation websites.