We use IXL.com. This site offers online practice and is standards based according to your state. You click on your state and the standards for every grade (up through 8th grade) for your state are already organized for you. You get usage info (how many minutes practiced, how many problems attempted), and kids earn virtual prizes as they complete a topic (a campfire picture that glows and moves, a soccerball icon, etc), among other things.
You do have to pay for it, but it seems a nominal fee for individuals. Not sure what the classroom/school licenses look like. You can try it for free as a visitor for, I think, 30 minute periods at a time.
I mentioned sumdog in my post above - since then I have written these instructions for students (on a new blog for students).
Also seriously worthy of a mention is Manga High - not just for the games but the Prodigi Quizzes - for schools with an account (and Manga High is now completely free in many countries). Teachers can set challenges for their students and see who has tried what. I included it in my Top >10 Mathematics websites recently. Scroll down to Games.