First the Khan Academy ("my goal is to cover everything") and now Number2.com, which C. found while I was in Minneapolis, attending msmi2010.
Number2.com is the only website that offers students access to comprehensive free online test preparation courses for the SAT, ACT, and GRE. How do we do it? Number2.com earns revenue from sponsorships and licensing.
The company was founded by professors and graduate students who wanted to make high quality test preparation universally accessible. In 1999 Eric Loken and Josh Millet were graduate students studying for their PhDs at Harvard University when they began teaching free SAT prep classes in the Boston area. Seeking to expand their services to the Internet, they teamed up with Vincent Crespi, a professor of physics at Pennsylvania State University, who had created a test prep website called Number2.com in 1995. With the help of a great technologist, Filip Radlinski, they launched a new version of the site in September, 2000. Eric and Josh now also run an employee testing company.
Since 2000, over 2 million students have enrolled in Number2.com's online courses, and the web site now receives over 50 million hits per month. In 2002 Number2.com was acquired by Xap Corporation....
C. likes it because when he can't solve a problem, Number2 gives him a hint instead of the answer.
In theory, the text prep books could do that, too, but they don't.