This page converts currency. When working with conversion problems what better thing to convert than money. It could be a great site if you wanted to make an interdisciplinary project involving traveling.
If you are a math teacher and you have not heard of Marcy Cook then something is wrong. She is a very famous person who has developed a lot of creative ways to use mathematical concepts in the classroom.
This is a site that a math teacher could use as it has polls. You could create your own polls to have students complete for class or use polls already on the site. You could creat pie graphs or work with percents by taking the info off the site.
This is the infamous illuminations website. It has so many applications that I could not possibly fit them all here. NCTM has done a lot of work to include many different concepts here. I would recommend that you visit this site and explore the games and applications.
This is an excellent site that allows you to clip information from the web. There are programs that cost money will accomplish the same thing but with the use of this site you can clip and save articles, and other information into a kind of scrapbook of information.
This is Dr. Reiter's homepage. This is without a doubt one of the best sources of mathematical information that you could want. He not only has information about his classes but he has links to some of the best websites.