Ambleside Online Math Options

There are many good math programs out there, and which you choose will be determined by your own math background, comfort level and time to invest teaching math. Those who are comfortable teaching math seem to prefer hands-on programs that use Cuisenaire rods like Miquon, while others prefer a less teacher-intensive program like Math-U-See. Math programs can follow a mastery or spiral formula (repeating principles and building onto itself over the years, which is fine as long as the steps are mastered at each stage of the "spiral"). The following programs are very popular among Charlotte Mason homeschoolers, but this isn't an exhaustive list of effective options.

Developmental Mathematics by L.George Saad; 16 workbooks focused on individual math concepts
Making Math Meaningful by David Quine; a math program developed by a CM family. "Easy for the mom." Six levels, plus algebra and geometry.
Math U See by Steve Demme; includes teaching videos, manipulatives and consumable texts
Miquon Math is a primary math program based on Cuisenaire Rods that provides a solid start in mathematics and is great for hands-on learners. It requires a fair amount of parent involvement and the teachers' materials can be slightly confusing, but there's a yahoo support list that can help, especially when starting.
Ray's Arithmetic is a solid no-frills set of books for grades 1-8 recommended by Ruth Beechick. This is the math used in the 1800's when "having an 8th grade education" meant being proficient at skills that our high school graduates sometimes lack. It's affordable ($100 complete) and develops skill at mental math, but is teacher-intensive. There is a yahoo support group
Singapore Math uses the methods that have helped Singapore obtain high international math scores. May need supplementing with another math program.
Right Start Mathematics by Activities for Learning is based on the abacus (more teacher intensive) for grades 1-8.

For more math help, suggestions and comparisons of curriculums, see:
Homeschool Reviews
Cathy Duffy Math Reviews (reviews of some programs aren't online, but must be read in her book, which is for sale from her website.)
Julie Brennan's Living Math has a CM focus
MEP (Mathematics Enhancement Programme) - this curriculum is free to anyone anywhere in the world. You can download the practice books, lesson plans, and answer keys (for upper years) through the University website. if you have questions about how to use the site, there's an email support group. Password required to use the site, but homeschoolers in the US have had no problem obtaining one when they ask.