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Sign up for free material by Cambridge English:

http://www.cambridgeenglish.org/exams/ielts/preparation/vocabulary-and-writing/ 

Taken from the above page:

With our free Language for IELTS learning materials on Cerego, you can learn, use and remember vocabulary for your IELTS test or academic study on your computer, tablet or smartphone.

We’ve worked with Cerego, an online learning platform, to help you increase the vocabulary you know. By using Cerego’s adaptive learning system it will tell you exactly what you need to learn and help you to improve your English through short, frequent activities every day.

You can choose what you want to learn:

Learn and remember IELTS vocabulary: Education
Improve your range of vocabulary on the topic of education. Learn useful expressions to use in all parts of the test.

Learn and remember IELTS vocabulary: Graph language
Learn useful phrases to describe trends in graphs. This is especially useful to help you feel more confident in IELTS Academic Writing part 1.

Learn and remember IELTS writing: Linking ideas
Learn ways to link ideas in a text. This is especially useful for IELTS Academic Writing parts 1 and 2.

These materials are part of a free trial and are available for you to use until March 2016.