Jim Breen's JDic is huge. It is particularly good for technical words and geographical names.

The Saiga Kanji Dictionary offers detailed kanji descriptions including stroke-order, readings and audio pronunciation files.

Jeffrey Friedl's dictionary uses the same database that Jim Breen's dictionary is based on, but has a different look to the search results.

Also useful are Handwritten Kanji Recognition and Kiki's browsable kanji index.