Compleat Lexical Tutor
This site, created by Tom Cobb, of Quebec University, Canada is a vast and comprehensive resource for vocabulary learning (not only in English but also French, German and Spanish as well). Despite difficulties in using it (see below) it does offer some very good tools for learning.
The most useful part is probably the Levels Test (Productive) which is found under Tutorial, Test your lexis on the front page. This allows you to test and re-test yourself at different levels. The tests are simple to take and give immediate feedback. Also very useful is List_Learn, just under Tutorial, Test your lexis. This lets you look at lists of words of differing frequency, including the AWL, and to see the definition of a word or see it in a concordance. It has a search engine: you can type in a word and search for it within any of the lists.
Navigation is difficult. The front page does not give enough information about what each link will take you to. You need to explore, and much of the site is more relevant and accessible to teachers than to students. Some of the exercises lack clear instructions.
A test on the most common 14,000 words in English, divided into levels of 1,000 words each. (Called 1-14k levels test). In general it is a great site for those interested in researching vocabulary as it has many tools that allow you to do that.
Instructions are not always provided and pages are sometimes confusing.