WordWeb is a one-click English thesaurus and dictionary for Windows that can look up words in almost any program. It works off-line, but can also look up words in web references such as the Wikipedia encyclopedia.
Speak English Like an American is a great book and CD for improving your American English (http://www.languagesuccesspress.com). The company just released a new iPhone app Speak English Like an American which will help you master American English words, idioms and expressions. Very cool and helpful! For more info:
I would suggest you vocabulary.com, which helps you learn words at your pace by asking questions in an interesting way. The questions asked are adaptive and the scores are tracked. The site claims
"Vocabulary.com is the easiest, most intelligent way to improve your
vocabulary. It combines an adaptive learning system (The Challenge)
with the world’s fastest dictionary (The Dictionary) so that you can
more quickly and more efficiently learn words. "