I prefer the Chinese online free dictionaries available, but if you need to use an offline translator, this is the conclusion of my research of English-Chinese dictionaries for English speakers:
Free dictionary: http://www.lingoes.net/
In the same web I actually recommend the next dictionaries between the more than two hundred available (Simplified Chinese)!
Maybe it is important to notice that both dictionaries have pinyin pronunciation included.
And just in case too: