Joan Holub's 100th Day of School Website
http://users.aol.com/a100thday/index.html
Click at the bottom to get ideas to celebrate and a listing of 100th day books.

10 x 10 Jellybeans
http://www.miamisci.org/ph/jbean100.html
This Miami Museum of Science interactive and visual problem illustrates taking 100 jellybeans and thinking about how much space 10 times that would be, then 10 times that and so on.

100th Day of School Celebration
http://www.siec.k12.in.us/~west/proj/100th/
Find out what other teachers are doing by clicking on the Classroom Activities button.

Kidproj
http://www.kidlink.org/KIDPROJ/Math/100days.html
Although this site hasn't been updated since 1996, it has some great ideas to use and many of them involve math.

100th Day of School
http://webtech.kennesaw.edu/jcheek3/100thday.htm
Here is a collection of LOTS of great sites to activities on different topics and levels for celebrating the 100th day of school. 

Celebrate the 100th Day in 100 Ways
http://www.education-world.com/a_lesson/lesson149.shtml
Education World offers you the best 100 ideas we've found for celebrating the 100th day of school. Try one -- or 100 -- with your students!

100th Day of School
http://www.vickiblackwell.com/100th.html
Here is another wonderful collection of ideas and activities for the 100th day of school.

Math/100th Day
http://www.mrsalphabet.com/100thday.html
Check out these math ideas for the 100th day celebration.