There are several sites you can use to learn a new word everyday and hopefully improve your vocabulary:
Merriam-Webster Online: Word of the Day -- Besides the definition, they include a sample sentence and a link to hear the pronunciation. You can either subscribe by email or RSS feed.
Dictionary.com: Features the meaning of a single random word, its pronunciation, etymology and examples of word usage.
Wordsmith.org: A daily newsletter with A.Word.A.Day for people that delight in the joy of words.
And the ultimate word site -- The Oxford English Dictionary: sign up to have the OED's Word of the Day e-mailed to you or receive their RSS feed.