Hi Sarah, not sure which grade you need, but you can check out my Livebinder of resources for all elementary school subjects. Many links in math are also good for middle school students.
Have you looked at TenMarks?
Go to mathlanding.org. It is a new website of vetted elementary math resources aggregated from the web. And everything is free. The search engine is easy to use and there are lots of ways to find resources--advanced search is great and you can browse by common core standard too. Be sure to check out the "collections" as well. I think you'll love it!
I will suggest you to check this site to learn more about math queries http://www.1to1tutor.org/, It will really help you.
Have you examined "Math Tricks"?
These tricks have mathematical calculations that promote using your brain, with sometimes help from a calculator, a pen, a paper and a friend.
You should try https://www.matific.com. It focuses on building mathematical foundations and it is free to schools.
Natalie mentioned Sumdog - that's definitely worth a look - good practice.
I'm a fan of WolframAlpha for all ages - even little ones as I think it can be so useful for students to check their work. I have included WolframAlpha's blog posts on some queries younger students could try on this page which also has other sites you may find useful.
My school just began using IXL. It isn't free but they have more subjects than just math and it's really comprehensive. The kids can move through it at their own pace and if they make a mistake, the site goes back and reteaches the mathematical concept.