i'll put in a(nother? i can't remember!) plug for the Australian program, Mathletics. it's not totally flexible (ie - they do ask you to pick a grade) but there are always easier/harder options available for every lesson. it also doesn't force them to crank out a billion versions of the same type of problem, which is a mercy for a kid who doesn't need a ton of reinforcement. even my kid, Captain Eyeroll, can stick out the 10 Qs in a lesson and the 15 Qs in a test without deploying too much sarcasm. she also likes that she can skip straight to the test if she feels confident.
i'm using Mathletics as a loose guide for the math portion of our homeschooling program - it certainly exceeds our regional expectations for Grade level. we do take lots of detours, but as far as the nuts & bolts of what we need to cover, it's doing us proud.
