- nciku: Nciku is my first stop when looking up a Chinese word. I've found that they simply cover more of the words I need than the other dictionaries on this list. They often supply numerous example sentences, which also puts them ahead of most of the other dictionaries here.
They supply the pinyin for the words you look up, and pinyin is supplied for example sentences via a pop-up when you hold your mouse over them. The latter is kind of a pain because you can't copy and paste it and you can't see it without doing something, but you can always copy and paste the Chinese word itself into the search field to get copyable pinyin.
Example sentences can be pronounced via a pretty natural text-to-speeh program which goes beyond the simple character-by-character pronunciation of MDBG, but there's no pronunciation button available for the definitions.
Another very convenient feature is that you can write characters by hand to look them up—a great feature when you can't recall the reading.
If you're curious about what "nciku" means, see here and here.
- Dict.cn: My second stop is usually Dict.cn. More aimed at the Chinese learner of English, Dict.cn's strong point is their example sentences sourced from the net, similar to ALC for Japanese. There is no pinyin and no pronunciation of Chinese words.
- MDBG: If you dig around the internet for Chinese-English dictionaries, you'll surely run across some of the numerous online dictionaries based on the CEDICT project. But the site serving as the caretaker of the CEDICT project and one of my favorites Chinese-English dictionaries is MDBG.
MDBG has a large breadth of vocabulary and always provides the pinyin. MDBG can even color code characters and pinyin syllables by tone. Audio pronunciations are provided, but they pronounce each character as if it were alone, i.e., they don't take into account systematic tone changes based on the previous tone. Like nciku, you can also write characters in with your mouse to look them up.
- iCIBA: A lot like Dict.cn: aimed at Chinese learners of English, strong point is example sentences (which also seem to be sourced from around the net), no pinyin, and no pronunciation of Chinese words. They tend to have more, but simpler, example sentences than Dict.cn, and also will serve up related encyclopedia entries.
- Reverso: Reverso has pinyin but no pronunciations. The word coverage generally seems to be less than the others above, but they are an additional option that I turn to occasionally.
This post was updated on October 6, 2009, to clarify the name of nciku and to note that nciku does in fact have pinyin (thanks to doviende of Language Fixation for the clarifications).