Lexxica provides resources to determine how much vocabulary a student knows and find gaps in their knowledge. They also claim to be able to accurately assess which words a student already knows.
The site is difficult to navigate and prone to crash. If all goes well, try the V-Check to find out how vocabulary you know (works for native speakers, too) and V-Admin to process your data. There are also flash games, free graded texts and flash cards.
I would be interested if other members here tried it and posted comments.
Lexxica: URL - http://www.lexxica.com/
In the last week I have contributed a graded headword analysis script to the Text Analyser in the Members' section (link: http://www.usingenglish.com/members/text-analysis/).
It is based on work I did recently to build a text editor for authoring graded readers with Dr. Robert Waring at Seishin Notre Dame College in Okayama, Japan. Although much simpler than the text editor, it allows teachers and students to run short texts (up to 100,000 characters) against two popular graded lexicons, provided by Dr. Paul Nation at Victoria University, Wellington, New Zealand. A detailed description of how to use it can be found off the Help tab when you make your analysis (link: http://www.usingenglish.com/members/text-analysis/graded-analysis-help.php).