Drop Some Knowledge: Khan Academy
As I'm currently studying for the MCAT (wish me luck!), I get really bored of just reading text books day after day. So to supplement my studying, I listen to free short lectures by Khan Academy professor Sal. He really knows what he's talking about, and is a great resource for clear explanations of SN2 reactions in organic chemistry, for example.
He teaches a ton of topics, from calculus and physics to lectures to help you understand the current economic crisis and famous historical events. He also has videos on studying for the SAT and GMAT exams. His videos are short, about 10 minutes each, which make them easily digestible. So if you need to brush up on just about anything, check out the Khan Academy YouTube channel here. And for a complete list of all the topics they cover with links to the corresponding videos, visit their official site.