College Board has been offering daily SAT questions by email for awhile now (and there's no lack of individual test questions online either), but what's useful is a full test, with questions of varying difficulty and a time limit enforced. Taking it on paper is important too; your score when you take the test in front a screen versus when you have paper, pencil, and a calculator won't be the same. There needs to be dozens of these tests -- I found myself getting better each time, but re-taking tests isn't helpful because I simply remembered my mistakes rather than improving my capacity to anticipate mistakes.
This is not at all to downplay the amazing work Khan Academy has been doing! I think this will be a big deal for a majority of high schoolers.