Wolfram|Alpha isn't exactly an encyclopedia, and it isn't exactly a search engine. The brainchild of Stephen Wolfram, this utilty allows you to compute answers from information gleaned from the web. It is especially useful for measures and conversions, mathematics and computing, and numeric sequences such as population, stock prices, or exchange rates over time.
Wolfram|Alpha is free and you can find the mobile site at http://m.wolframalpha.com/. Definitely bookmark-worthy!