If you are planning to take the SAT in January, Saturday, December 28, 2012 is the last day you can register without paying late fees. This is also the last test for which a photo ID is not absolutely mandatory.
For students taking the SAT in the United States and Canada, The Right Path also highly recommends ordering the Question Answer Service. The QAS is only available three times per year and can be ordered from the registration site or up to five months after taking the test.
For more information about the QAS, click here.