WolframAlpha.com

WolframAlpha media type="youtube" key="oORzFifLFGI" height="275" width="312" Answer questions, do math, instantly get facts, create plots, calculators, unit conversions, scientific data and statistics, help with homework—and much more.

Advantages: Content variety is absolutely absurd Always on-call Never down Helps "Do" homework Good for extensive research products

Weaknesses: Costs $ on iPod, iPhone, or iPad Can get addicted. Seriously.