boellWIKI

OALD Oxford Advanced Learner's Dictionary: http://www.oxfordadvancedlearnersdictionary.com
(former link: http://www.oup.com/elt/catalogue/teachersites/oald7/lookup?cc=global)
→ the standard tool you know from your English lessons

MED Macmillan English Dictionary: http://www.macmillandictionary.com/
→ an EXCELLENT (!) online dictionary, available with two options: British or American English; moreover, you may listen to audio examples to learn a word's pronunciation, and a Thesaurus tool allows you to find synonyms

OCD Oxford Collocations Dictionary Online: http://llohe-ocd.appspot.com
a collocation is a combination of words in a language, that happens very often and more frequently than would happen by chance; thus, this dictionary tells you, e.g., which words are normally used with each other (to make, do + homework; or: to gain, achieve, attain, reach + a goal)
→ very helpful to make your English sound more natural

less prominent dictionaries:

CALD Cambridge Advanced Learner's Dictionary: http://dictionary.cambridge.org/

LDOCE Longman Dictionary of Contemporary English http://www.ldoceonline.com/