The Oxford 3000

http://www.oup.com/elt/catalogue/teachersites/oald7/oxford_3000/?cc=gb

This page gives a list of useful word lists complied by Oxford University Press Dictionaries – the most frequent 3000 words in English; useful words for learning language; the most common words in science, arts and business.

Strengths
It contains the 3000 word profiler – a device that allows you to check a text to see what percentage of it is made up of the 3000 most common words in English.

Weaknesses
The words lists are just that: lists, without any exercises. They are not hyperlinked to a dictionary definition, either.