We have used the following:
IXL. DS7 did some self-teaching using this, bouncing around levels and skills, at age 5.
ALEKS. We were unimpressed for various reasons but it filled a need at the time and allowed speeding through a year or so of curriculum for hole-checking purposes.
EPGY. This seems all right for quality of instruction, but the software on our machines was buggy as heck. Never got into it for this reason, and in any event we prefer books.
Life of Fred. There's nothing really to recommend against this, but it's only enrichment, not a curriculum. We've also used bits of Ed Zaccaro Challenge Math a while back, and many other sources of problem solving enrichment. Raymond Smullyan and similar logic puzzles are another fun type of challenge.
Singapore Math. There are some off-brand "Singapore 'Math Practice'" workbooks sold on places like Amazon, but you have to make sure to buy the actual system if you want it, not copycat brands. The actual system uses teaching / home instruction guides, textbooks (important), workbooks (not so important for a child who learns quickly, in my opinion), and extra practice in the form of Challenging Word Problems and Intensive Practice books (important, but you would generally only need one or the other, since both are pretty challenging at the right level). It's my belief that you can get by without the teacher's manual / instruction guide if you know what you're doing with math, but it's cheap so you might want to get the first one just to see how you like it.
(Here, by the way, is what a "Singapore 'Math Practice'" workbook looks like, with branding such that people tend to confuse it with the actual Singapore Math curriculum:
http://www.amazon.com/dp/0768240018This is what a workbook from the Singapore Math curriculum looks like:
http://www.amazon.com/dp/9810185014 )
"Geometry", by Harold R. Jacobs (the older non-cartoonish version). I can't recommend this enough.
Algebra, Structure and Method Book 1, by Brown, Dolciani et al. I think this is also a great resource, and DS has learned a lot from it.
I also recommend taking a look at Alcumus and Art of Problem Solving; we just haven't gotten much into them beyond dabbling in Alcumus yet.
