While we don't have an ideal test-like practice environment yet, I hope that we will be able to provide one in the future.
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.