I compiled these links for a college EFL class I am currently teaching. The students’ first language is Arabic, so an Arabic dictionary is included among the reference tools.
I will include a warning at the beginning that I left until the end on the handout I gave my students: I do not recommend using Google Translate for anything that you will turn in for a grade. Google Translate gives too little context for distinguishing different meanings and uses of a word, so stick with a traditional dictionary (even online) if you need to be sure that your translation is correct.
Dictionaries and Reference
Aljazem Arabic-English / English-Arabic Dictionary
http://www.aljazem.com/
British Council Quick Grammar Reference
http://learnenglish.britishcouncil.org/en/quick-grammar
Collins English Thesaurus
http://www.collinsdictionary.com/english-thesaurus
Longman Dictionary of Contemporary English Online
http://www.ldoceonline.com/
Merriam-Webster Learner’s Dictionary
http://www.learnersdictionary.com/
Merriam-Webster Visual Dictionary Online
http://visual.merriam-webster.com/
Research and Writing
Purdue Online Writing Lab
Information on research, writing, and citation; includes a special section for English learners.
http://owl.english.purdue.edu/owl/
Writing Tips from American University’s Academic Support Center
http://www.american.edu/ocl/asc/Writing-Lab-Writing-Tips.cfm