musictheory.net

-Take advantage of this awesome site, which has sections for Lessons and Exercises in music theory. They also have an app that you can purchase for your iPad, iPhone, or iPod Touch.

tobyrush.com/theorypages

-This site has a collection of PDF files of music theory aides. They take one subject and summarize the basics on one page, so you can think of them as cheat sheets.

-My particular favorite is their Circle of Fifths

www.teoria.com

-Another excellent site with ear-training exercises, music theory references, articles, and tutorials.