Chinese-English CEDICT (dictionary from http://www.mandarintools.com/cedict.html): DictionaryForMIDs_2.7.0_EngChi_CEDICT.zip (3248 kB)
Requires cell phone/PDA with a Chinese character set.
English -> Chinese: enter English word as usual
Chinese -> English: enter Chinese characters (both traditional and simplified work) or pinyin with tone numbers (e.g. "ci2 dian3") or pinyin without tone numbers (e.g. "ci dian") or pinyin with accents (e.g. "cí diǎn").
Some tips on searching Chinese: