The UIUC Library has a large collection of print dictionaries. To find a monolingual English dictionaries, search the Library's Primo Catalog using the subject heading English Language Dictionaries and selecting "subject" or searching using the keywords English Dictionary.
The American Heritage Thesaurus for Learners of English by 
Merriam-Webster's Collegiate Dictionary, Eleventh Edition by Electronic Books
Wallwork, Adrian. English for academic research : vocabulary exercises. New York : Springer, 2013.
Print Books
Vocabulary flash cards for the new naturalization test [2009]
Web Sites
Academic Word Lists - compiled by a scholar in New Zealand, these lists of commonly used academic words are organized by usage frequency.
Academic English Cafe - there are vocabulary quizzes as well as spelling, writing, and grammar quizzes. The writing prompts may also be useful for practicing persuasive and descriptive writing skills.
Word A Day - a new, unusual English word is posted each day with a definition, audio pronunciation, and example usage. Great for native speakers and advanced language learners alike!
200 Vocabulary Exercises - visual exercises in vocabulary from the University of Victoria English Language Centre.
WordSift is a new online tool for visualizing new vocabulary. A review article in Education Week describes how the tool works.
