Wiktionary

"English-language Wiktionary is a collaborative project to produce a free, multilingual dictionary with definitions, etymologies, pronunciations, sample quotations, synonyms, antonyms and translations. ... started on December 12, 2002, they currently have 419,951 entries in 389 languages.

Did you also know that Wikipedia is a great acronym decoder?

Wikipedia provides a listing of what various acronyms can mean.
i.e. ROI = http://en.wikipedia.org/wiki/ROI
or CE = http://en.wikipedia.org/wiki/Ce