E.ON Energy - Register your Tesco Clubcard and get 1 point for every £2 spent - Money for nothing!
Just registered my clubcard with E.On and I get 1 clubcard point for every £2 spent. This is pretty much money for nothing every year... it all adds up and it will take you 20 seconds to register.
Also check out if you are on Economy 7 or Standard tarrif, they can tell you instantly if you are on the right one for you. I have just saved over £200 per year by switching to standard... more cash i have clawed back and it hasn't cost me a penny.