Try this! A wonderful tool to use for reading on the web. VoyCabulary makes the words on any webpage into links so you can look them up in a dictionary or other word-reference-site of your choice, by simply clicking on the words.
ESL Slang Page - Dave's ESL Cafe
Merriam-Webster Dictionary Online
Vocabulary Lists - Idioms, Phrasal Verbs and Slang - Activities for ESL Students
Academic Word List and Practice Exercises
The Academic Word List is a list of words which appear with high frequency in English-language academic texts. Learning these words can help you to understand general academic English. You can find out more about it and do practice exercises at this web site.
This is a site which lets you improve your English vocabulary. For every answer that you get correct, they donate 10 grains of rice through the UN World Food Program to help end hunger.