The standard workhorse dictionary seems to be the revised edition of the Chinese-English Dictionary put out by the Foreign Language Teaching and Research Press (外研社). It's fat, relatively inexpensive, and has a fair number of good sample sentences, basic chengyu, and the names of rare diseases.
The two-volume, 3500-page 汉英大辞典 published by 上海交通大学出版社 is a decent addition to the reference shelf.
There's no shame in using a thesaurus when you're translating – how else are you going to come up with four synonyms for "expansive" in the space of three minutes? A particularly nice one is the Collins Thesaurus, notable for grouping words under thematic categories – you first look your word up in a disambiguation index in the back, and once you've got the proper meaning of the word, you're off to a full page or two of likely-related words. Very satisfying.
A relatively unknown but highly useful website is the OneLook Reverse Dictionary. The idea is that you're translating along, and you suddenly realize that there's a perfect word for the thing you're trying to render into English, but just won't come to you. So you go to the Reverse Dictionary, type in some words describing the word you can't remember ("the hard outside of cheese"), and the Reverse Dictionary gives you 100 words, most of which are garbage, the first of which is "rind".
Nciku/N词酷 is a popular, free, convenient and frustratingly basic online dictionary.
Wenlin, featuring 10,000 Chinese characters and nearly 200,000 words and phrases, is the most flexible C-E dictionary/translation software I have encountered. For beginning users, the stroke-order guides and easy transition between traditional and simplified characters are invaluable; for advanced users, the ability to add to and customize dictionary entries is pure joy. Well worth the $199 USD sticker price.
Hanzim - at $34.95 USD, an affordable alternative to Wenlin
Geeks will be thrilled to find the cjklib library, a Python-based set of databases and lookup functions that allow you to do pretty much anything you'd like to with a Chinese character: discover its stroke order, decompose it into parts, treat it as a radical and find out what characters are made from it, learn how to pronounce in pinyin/Wade-Giles/Korean/Shanghainese… And, of course, do all that backwards.
辞海, literally "Sea/Ocean of Words", is a must for any serious translator. It's the OED of Mandarin Chinese.