An exceptional source of Word Information that provides extensive word families in addition to the word entry that a person is looking for.
Another significant source of words that are grouped in their topical or subject areas is available at this Get Words site.
A good magazine that provides superior vocabulary development is The Atlantic Monthly, especially the Word Watch which will answer many questions about English-word usage.
An extensive discussion about words and English usage is worth seeing at Collins Cobuild.
If you have never heard about the Devil's Dictionary by Ambrose Bierce, then take a look at the definitions offered at this site. If you have experienced this special dictionary, then you know that you are in for a satirical adventure. For those who aren't acquainted with this publication, it has nothing to do with devil worship.