Bilingual dictionaries are great. English learner dictionaries are better! These dictionaries have been designed with English learners in mind, and as such provide extra learning tools for pronunciation, verb types, basic grammar structures, and more. These dictionaries are also designed with standard test syllabi in mind so that learners are guided towards the TOEFL, IELTS or any of the Cambridge Suite of Exams (PET, KET, FCE, CAE and Proficiency).
Longman is the very best for students learning English to study, live and work in North America. This dictionary provides lots of handy reference materials, special in-depth looks at issues such as idioms, phrasal verbs and more.
The American Heritage® Dictionary for Learners of English is especially designed to suit the needs of ESL students. With an up-to-date word list and definitions adapted from The American Heritage® Dictionary databases, abundant sample sentences and phrases, and an easy-to-use alphabetic pronunciation system all provide an excellent learning tool.
The standard in British English, the Cambridge Advanced Learner's Dictionary provides the ideal tool for English learners who wish to take any of the Cambridge advanced exams (FCE, CAE and Proficiency). Dictionary includes a learning CD-ROM with helpful resources and exercises.
For lower level learners, the Oxford Elementary Learner's Dictionary of English provides key English learning resources especially suited for British English classes.
Oxford's advanced learner's dictionary is suited to advanced level learners of British English. Unlike most North American resources, Oxford does a great job of providing comparisons between standard North American and British English. This dictionary is excellent for those who wish to study English as a global language.