http://www.macmillandictionary.com/
http://www.oxforddictionaries.com/
http://www.merriam-webster.com/
http://www.thefreedictionary.com/
The tool Word Neighbors (http://wordneighbors.ust.hk) developed by The Hong Kong University of Science and Technology is an excellent way to learn vocabulary. The tool allows you to look up common word combinations (collocations) and review how they are used in real situations. You will often be advised to use this tool throughout the Specialization. A tutorial on how to use Word Neighbors can be found here.
