Just go to http://www.free-dvd-club.com/
(or click the link) and choose your free DVD. You have to do a five minute survey (just say 'no thanks' to everything - though you can get 4 free o2 sims too which is handy)
You do get a lot of email spam so my tip is just use an old email addy (or set up a temporary one at mytrashmail - very handy site!) You DON'T get any postal spam at all. You can only do this once per address (though it seems to reset after a few months as I've done it 3 times in the last year). The majority of DVDs are full retail boxed versions but the 'night of the living dead' which I ordered for my dad came in a sealed small snap shut case. P&P is free, no card details to enter.
List of free DVDs in the next post (there are also a few books too).