How can I generate a list of missing parts for incomplete sets? Does anybody know if there is a website where I can record my incomplete LEGO sets?

That way I could generate a list of parts I would require in order to complete them.

I currently use Brickset, however that only allows the user to record completed sets.
Rebrickable.com offers this service. First add the complete set to you list, then view it on the set page like this http://rebrickable.com/sets/42029-1/customized-pick-up-truck-technic-2014 At the bottom of the first panel you'll have an option to "Add to my parts list" from then you go through the list of parts and remove the ones you are missing.