I recently tweeted asking for resources for math and before I knew what was happening people tweeted math, math, math!!
HotMath is a subscription site for middle and high school math. They do have some good free things to check out and the link will take you to several neat games. Explore the site and find some of the other free activities to use with your students. Thanks to Eric Sheninger-Twitter name-NMHS_Principal.
Twitterer rmbyrne, teacher from Maine shared MathTrain TV. This site has math videos made by students...kids teaching kids!! The nice thing about this site is the videos do not appear to be from YouTube so you should be able to show them in school. There is a download button so I think you can download them and share them without having to worry about the problem of streaming video.
Now sharnon007 has so many great ideas that I am overwhelmed by all her resources. She's come through with many great sites and she keeps sending more and more! Here's a few...
Smartkiddies Mathematics is from Australia. Although you have to sign-up, the site is free. I haven't looked at all the activities but what I have seen is good! Lessons go from Kindergarten to Grade 6.
Math Doodles has some interesting games. Click on Web Demos to find them. Right now I think the site is in beta. That means it is new and still being tested. Sites in Beta are typically free but may later have a fee attached to the subscription.
Students don't always need to be on the computer. Sometimes the good old worksheets come in handy. The only problem is that you need to check the papers! Mathmaster doesn't do the checking but this site can help you make customized math worksheets or provide you with premade worksheets.
MathTwo looks like a new site but it has some great games to practice skills. Click on the title to link to the home page where you will find the activities. Each will give a brief overview and then give you a choice of skill level. After you check out the games, click on Cool Links to see a list of interesting sites.
If you want me to find specific skills, just let me know and I will send you activities to align with the CSOs you are teaching.