I speak, of course, of Khan Academy.
I've been recommending Khan Academy to parents looking for extra help for their kids on math (and I would stick to the math topics... he gets a bit fuzzier when he gets outside that, in my and other people's opinions. The test prep stuff is okay).
It's not just for kids or people in school -- if you want to brush up on some math, it's a good place to go.
Salman Khan got known through his YouTube videos first, but since then he's had a website built out where one can do practice problems and set up a logical sequence of progression through various topics. And, like many of the learning sites, you can earn badges. You can get a coach to kick you in the ass, if you want... I mean, uh, to watch your progress and to recommend stuff to do.
I signed up two of my kids for the site (and I'm their coach). They've enjoyed it (and I even use it as an enticement for them to get their regular math homework done -- finish that, and you can do some Khan Academy).
For adults, the areas I recommend isn't so much the exercises and drilling, but stuff like the Applied Math area. And then there's Vi Hart's videos. But more on Vi next week, as I'm going to do a little field work for that one.
There have been complaints about Khan's approach, and also about errata... thing is, I think his error rate is not that bad, and it helps to realize that all the educational material you ingest likely has some inadvertent errors in there (I will not speak about the deliberate ones). I have been involved with math text editing before, and yowza, the amount of errors before edit is amazing sometimes (and then, when I was a math text reader with this outfit, they had to explicitly tell me not to fix the edits in the record... which was driving me nuts).
With math, you can actually check stuff directly, which is why I didn't mind errors there so much. It's so much harder to talk back on these sorts of things in history class. (Yes, I used to get in trouble for disputing such things as Georgia being a debtor's colony. Middle school teachers are not all that interested in getting into scholarly debate on these topics.)
The real strength of Khan Academy is that you can keep repeating the videos, and the exercises, until you finally get it. You go at your own pace. And there's some fun math stuff in there as well, not just "serious" math.
ADDITIONAL: LeBron has some questions for Sal.