This site is the absolute favorite in our house: www.noradsanta.org. I'm not sure how much is on here before Christmas Eve, but it is supposed to go back up for this year in a few days, so you should check it out.
Finally, this is definitely not recommended for a school environment or for younger kids, but Elf Bowling is pretty darn funny (available at www.nstorm.com). The original version (and I think maybe Elf Bowling 2) is the least offensive and free; later versions are more risque and I think you have to pay for them. Some parents may think they're ok for middle school/older kids at home; some may just want to play themselves.....
This year we are doing an ink cartridge race. The classroom winner gets to have free computer lab time and a holiday party. I'd like to list some good holiday websites for them to choose from but I have lost most of my favorites. These kids are 5 and 6 graders but I could use these sites for my kids here at home too. We love the holidays!