Northpole.com is a great site for kids. Scroll down a bit and you'll see a ton of options. Kids can write a letter to Santa or watch disco dancing Santa. You can even have an IM chat with Santa's elves. The site also has great options for parents looking for holiday ideas to keep the kiddies entertained while they're off from school.
Elfyourself.com has a Christmas theme but it will likely bring laughs the entire year and it just takes a minute to set up. You'll need to upload a picture of yourself and you'll be able to crop and adjust it so it fits nicely under the hat; then you'll be prompted to choose the kind of dance you want: country, hip hop, singing, traditional or, my personal favorite, disco.
At Portablenorthpole.tv the concept is to design a personal message from Santa to someone in your life who's been naughty or nice. Click on the start button and you'll go through a series of pages that will ask you for information like who the message is for, the recipient's name, age, etc.
So that's a very fun website and we have one more you should visit: Stjudesranch.org. The website is doing something very special; people donate their Holiday Cards and the Children at St Jude's Ranch put a new back on them and sell them the following year. The benefits are two-fold: customers receive "green" holiday cards for use and the children receive payment for their work and learn the benefits and importance of "going green".