Yellow Bridge is the best dictionary for learners with related characters and compound words, stroke-order animations and entymological information.

The MDBG Dictionary offers detailed character descriptions including stroke-order animations. They also have handwriting recognition and a very useful anotation tool.

The CEdict database is huge. It is particularly good for technical words and geographical names.

Dict.cn offers only simple translations; however, it does show usage in a range of sentences.

Other good dictionaries online include Zhongwen.com and the Chinese Character Dictionary.