There are many Chinese/English dictionaries online. Here are some with brief descriptions. If I am missing a good one, please leave a comment. All allow searching by pinyin or English.
- chinese-tools — Search by radical also. Usage examples. Audio for single characters. Thesaurus. “Sinogram” feature shows character’s radical, number of strokes, words formed from it, Unicode code point, etc.
- instantspeakchinese — Simple, 1-box search. Results include animated gif of stroke order and number of strokes; words beginning, or including, a character; frequency ranking. An online flashcard feature is a paid option.
- mandarinspot — Simple, 1-box search. Will display transliteration as pinyin with tone marks or tone numbers, or as bopomofo. Mobile-formatted version.
- mandarintools — Looks outdated now.
- MDBG — Search by handwriting (needs Java) and radical also. Mobile-formatted version.
- nciku — Search by handwriting (needs Flash) and radical also. Usage examples. Save vocab lists. Mobile-formatted version.
- xiaoma cidian — Simplified characters only. Browse by stroke count, frequency, HSK level. Character entries also give Unicode codepoint, frequency, HSK level, as well as links to words containing or beginning with the character and a link to Wiktionary. Choice of font.
- yellowbridge — Search by handwriting (needs Java). Audio for characters and words. Usage examples. Stroke diagrams (needs Java [problems on my computer]); Gives frequency rank, Unicode code point, input codes, words formed from characters, “Etymology Explorer” (character/radical composition), etc.