You probably already know of these sites, but just in case...
http://www.moneysavingexpert.com/shopping/
http://www.bobsfreestuffforum.co.uk/
The first is the most famous, and has details of free stuff, how to get almost anything else - insurance, power, phone calls and so on - cheaper. Also has loopholes, bargains, etc. The second link is purely for freebies, usefully sorted into categories; I haven't had to buy shaving foam, toothpaste or sunscreen since 2007!