I got a free energy monitor for signing up to paperless billing and signing up for reading my own meter (which I don't as I forget!) That was with Npower.
It clips round the meter tails and revealed that I was using about 3p per hour all the time…turned off a few things like the Wii and DVD player and now its just 1p per hour. Doesn't sound much but by usage has gone from about £1.50 a day to about £1. And when I get home from work I can tell if we've left an upstairs light on all day as the reading has doubled.
We bought a plug in one last year that tells you how much an appliance is using but its a bit crap and nothing like as good as one that does the whole house. (Edit: It was that maplin one!)