Here are some resources that will help you ace standardized testing and sound more sophisticated in your essays.

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InstaDefine.com - A really fast, live-search dictionary that predicts words as you type. Hover over anything to get more information about that word. Now you never have to leave the page.

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EasyDefine.com - A very convenient tool to define multiple words at once with tons of features including flashcard generator, quizzes, and worksheet generator. You can even create an account and store all your wordlists to use at a later date.

Other
Writing/Timed Writing Resources -- SAT/AP etc. + General

Good Examples For AP and SAT Essay
http://essayexamples.tpclubs.com
General Strategy for Timed Writing
http://www.jacobblog.tpclubs.com/2010/03/how-to-do-timed-writing-a-tutorial-version-0-2/

Really high quality notes to help with AP Classes/Tests:
APStudyNotes.org