This web site gives you access to all the various types of online dictionaries by Cambridge University Press.
This web site gives you access to the Merriam-Webster's Dictionary and Thesaurus.
Howjsay is an online dictionary that will pronouce evey word you type including suffixes.
Lex Tutors is an online concordancer in French, German, Spanish, and English that allows you to identify possible collocations for the words you search.