We've also used IXL.com to fill in gaps for things that aren't "core" math, like telling time, money, shapes, etc. It's a way to browse topics for a certain grade with very little time investment. I don't know how accurate it is but we have found things DS didn't know that we didn't realize were covered in that year.
You can do a certain number of free problems per day, which are about right to do a quick daily math type of thing. After a year of using it like that we did finally buy an account, but you don't really have to.
It's just okay but for what we use it for it's actually kind of great. The biggest challenge is keeping DS from wandering off to high school level topics.
