Britannica Mobile allows you to search and browse a subset of Encyclopaedia Britannica content from your phone.
The site at http://m.eb.com is open to all and does not require authentication. While optimized for iPhone, the site seems to work well with most phones.
Yes, even librarians use Wikipedia—judiciously. For quick facts, dates, and bibliographies it can be very useful. The mobile version of the English-language site can be found at http://en.m.wikipedia.org/.
Wikipedia is also available as a free app at the iTunes store and on the Android Market.
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!
