Free Online Dictionaries

Monolingual Dictionaries

1. Longman English Dictionary

2. Merriam-Webster Dictionary

3. The Free Dictionary by Farlex

4. Oxford Advanced Learner's Dictionary

5. Macmillan Dictionary

Illustrated Dictionaries

1. Merriam-Webster Visual Dictionary

2. The Visual Dictionary

Slang Dictionaries

1. Slang City

2. Urban Dictionary

3. Dictionary of Playground Slang

Dictionaries of Idioms

1. http://www.usingenglish.com/reference/idioms/c.html

2. http://www.idiomeanings.com/idioms/

Dictionaries of Phrasal Verbs

1. http://www.usingenglish.com/reference/phrasal-verbs/

2. http://englishpage.com/prepositions/phrasaldictionary.html

Dictionaries of Proverbs

http://www.syvum.com/proverbs/