In my experience, the Compleat Lexical Tutor is one of the more powerful online resources for teaching and learning English vocabulary. There are any number of tools on the site to benefit both instructors and students, including concordancers, frequency and grammar checkers, quiz and test builders, etc., all of which can be used together with established English language corpora (such as Brown) and word lists (such as the Academic Word List). As an instructor, I have taken most advantage from the easy-to-use cloze passage builder, as well as the "Text Lex Compare" option, which compares the frequency of words in a text of any length with a chosen corpus. One other positive and attractive feature of the site is its continuous updates--in the two years that I have been making use of this resource, teaching and learning tools have been added regularly.
Possibly the only downsides to the Compleat Lexical Tutor are that 1) it does not place a high priority on attractive physical layout and 2) it might not be immediately or obviously user-friendly to students. Instructors with some training in vocabulary teaching will benefit from the resource, but untrained instructors and especially ESL/EFL students will most likely require guided overview of targeted tools before using them on their own.
There is a stated requirement of Internet Explorer 5 and above, though those users of Firefox will find the Mozilla browser equally compatible.