onelook: A dictionary search engine, providing one point of search for all sorts of monolingual dictionaries, both general and specific.

whatis: An encyclopaedia for IT terms.

acronymfinder: A dictionary for acronyms.

Microsoft Language Portal: MS glossaries, style guides and articles on localization.

TDA Language Search:Language Search Engine put together by the Translation Automation Users Society (comprising Microsoft, Google, Adobe and more). Wealth of knowledge at your fingertips.

Wolfram Alpha: search engine one step beyond. Provides relevant info straight-ahead, instead of providing links to explore. Still in its early stages, but very promising.