The British Council has this nice website for ESOL learners
A Website for Studying English as a Second Language
YourDictionary is the easiest-to-use online dictionary. It is a dictionary you can understand with lots of tools to help you choose your words precisely, understand a word, avoid usage pitfalls and be confident in your use of words.
Common Errors in English Usage
Vocabulary.com has a number of lists, plus information about roots and suffixes