Eijiro on the Web (ALC)

According to several of my Japanese friends, this is the best online dictionary available. That said, it is far more useful to native speakers than non-native speakers. Still, this is a great “secondary” source to help when your primary source (e.g. WWWJDIC) leaves you unsure. Another strength is its sample sentences which really help figure out if a word is appropriate for your intended use.