Inside-Out (http://www.insideout.net/elessons) This website has hundred of articles from easy to difficult. Each article has comprehension questions and other exercises.
Breaking News English (http://www.breakingnewsenglish.com) This website has many articles and listenings about current news, from easy to difficult, including quizzes. A great website to practice all of your English skills!
New General Service List (NGSL) -
Quizlet Vocabulary Lists-
Vocabulary Website - SpellingCity is a great website to learn and practice vocabulary in a fun way.
Original General Service List - If you want to improve your vocabulary, but you don't know which words are good to learn, check out these words lists.
English Prefixes, Suffixes, Roots - Understanding basic word parts is a great way to guess the meanings of new words. Click here to download a list of the common English prefixes, suffixes, and roots (10 pages).
Vocabulary in Sentences - These sites let you type in a word and see a sample of sentences from textbooks or journal documents that contain that word. It's a great way to learn collocations in English.
AntWordProfiler (http://www.antlab.sci.waseda.ac.jp/antwordprofiler_index.html.) This free program lets you analyze an article to easily see which words in the article are from the 1st 1000 word list, the 2nd 1000 word list, the academic word list, and which words are not on any of the lists.
Learn Vocabulary Using Maclean's (http://homepage.usask.ca/~dul381/common/vocabexercise.html) Here is an exercise to learn vocabulary using the Maclean's magazine website and an online dictionary.
"Interesting Things" (http://www.manythings.org/it/sitemap.html) A general ESL web site with many activities to learn vocabulary.
"The Idiom Connection" (http://www.idiomconnection.com/) This is a great site for learning all sorts of great idioms in English.