I think cashback websites are a great way to earn extra cash, from buying things you would normally buy (especially at this time of year).
I have earned nearly £200 cashback over the past couple of years. Most of which comes from Boden
(over £100 in fact)

The main cashback websites are Quidco and Topcashback. Both are good, but I think that Topcashback is arguably better, because you get a 1% bonus on top of the cashback, and there are no admin fees (Quidco charge £5 a year, which is deducted from your cashback earnings).
If you haven't already signed-up, I really recommend it:
With this link I can earn £1
With this link I earn nothing
Also, if you 'like' Topcashback's Facebook page, they will donate £1 to Breast Cancer Research.