One of the most convenient dictionaries online. This is my default dictionary in the CE category. It has a clean interface, is easy to use, has handwriting recognition, stroke order and sound. Sometimes the English definitions are a bit inadequate, though (true for most dictionaries). Has a useful function where you can paste a lot of text and look up all the words in it.
More information about dictionaries here: 21 essential dictionaries and corpora for learning Chinese.