Google Books Search: Download free out-of-print books (ensure you check the "full view only" radio button which allows you to download complete books).
Old Maps.co.uk: Buy or see old British maps and see what your area looked like over one hundred years ago.
Money Saving Expert: Martin Lewis's huge site offers some great advice in these transient times.
speedtest.net allows you to check you actual broadband speed, or discover typical connections in your area if you're considering another internet service supplier.
UK residents who buy online should join to get cashback - there is no catch, except that you have to wait a couple of months before the cash is paid. Membership is £5 per year, but you don't pay anything until you earn more than £5.