If you want to improve your English in a business context you'll want to hear and read how English is spoken and written in the real business world. During my past three years of teaching business English I've made a collection of top sites for learning business English. They can be used for self study, private classes or group classes. See the list below:
1) TED - http://www.ted.com
Here you'll find over 500 lectures from top entrepreneurs, CEOs, professors, politicians, social leaders and philosophers who are doing things that change the world. Most lectures have subtitles (available in six languages) which you can turn on and off. Subjects range from technology to business to social change.
2) Randall's ESL Cyber Listening Lab - http://www.esl-lab.com
This is an amazing site for practicing your listening skills. There's material for beginners, intermediate and advanced level students. Each recording has excerises you can fill in. They use different accents and speak in a natural way, so you can really train your ear.
3) Really Learn 100 Phrasal Verbs for Business - http://www.filestube.com (search for "Really Learn 100 Phrasal Verbs for Business")
Dilys Parkinson wrote this practical and thorough guide to the most-used business phrasal verbs. For intermediate and advanced level students phrasal verbs are of key importance.
4) Learn English Today - http://www.learn-english-
This is a very practical and complete site. The business section is very useful and offers vocabulary, expressions and example sentences in categories like Marketing and Sales, Job Interviews, Business Letters and Meetings.
5) 1000 Most common words - http://esldesk.com (click on "Vocabulary" and "1000 most common words")
On average the 1000 most common words appear about 76% of the time in English articles and in conversational English this percentage is even higher. So know them well. ESLDesk.com is a great resource covering all aspects of ESL.
6) Inc Magazine - http://www.inc.com
The top magazine for small and medium sized business owners. Here you'll find the latest ideas on marketing, sales, managing people, innovation, finance, raising funding, business plans, success stories from entrepreneurs, etc.
Online Slang / Expressions Dictionary - http://onlineslangdictionary.
Here you'll find almost any expression or slang. The explanations are in English, so your English level should be at least intermediate.
Google Translator - http://translate.google.com
Translate any words or entire texts between dozens of languages. It's usually pretty accurate, but sometimes it mixes the different meanings of words or doesn't translate natural language well.
Dictionary.com - http://dictionary.com
The best free English dictionary on the web. It offers the most complete explanation possible and if you spell a word incorrectly you are offered a list of words that are similar to your input and you can find the right one.
Business word glossary / dictionary - http://www.washingtonpost.com/
A useful explanation of business words from one of the best newspapers in the USA, the Washington Post.