Jibjab has some great holiday scenes that you can insert your family/yourself into, I particularly like the Holiday Rock and Classic Movie scenarios but don't think I would ever use the Twisted Holiday ones ....still each to their own.
Capture the Magic is a site that allows you to upload photos of your home and receive them back with Santa realistically added in - not a free service but it's a small price to pay for extending the magic a bit longer for those not quite believing children.
Portable North Pole is probably my absolute favourite festive site, I just love preparing personalised messages from Father Christmas for people, adding in details that make the video special for them.
If you're looking for a video to play , this reading of The Night Before Christmas read by the Chichester Festival Theatre Company is just delightful.
Another wonderful video offering here, The Digital Nativity , how would the Christmas story have played out on the internet?
The Northpole is a lovely site to use with primary age children, packed full of activities and resources.
Activity Village has some useful festive printables, including bookmarks, jokes and 3D snowmen.
Norad tracks Santa is very special, great fun to login on Christmas Eve and watch the big man's journey around the globe
Last but not least TES 12 games of Christmas should keep young and old occupied whilst you get on with the shopping!
Hope you have found something here that you can use during December, hope your month is a happy one ;0)