My eg/pg son uses Dreambox at school, but my beef with it is that it costs money and it's a propriety-system. It's also largely repetition and drill based. Ditto for https://www.xtramath.org/ (which at least is free!).

Today, there's so many open source materials which are freely available and accessible online that I don't think you need to spend money today if you don't have it or want to.

Here's a more comprehensive guide to free math stuff online:
http://www.goodsitesforkids.org/MathandTellingTime.htm
http://www.goodsitesforkids.org/Misc.htm

We've used http://www.e-learningforkids.org/ and http://www.abcya.com/ as well. I like Marble math from abcya.com because it's more manipulative and not just rote. The National Library of Virtual Manipulatives (http://nlvm.usu.edu/en/nav/grade_g_1.html) has more challenging material too.

livebinders.com is awesome for education (math included).