Getting a good English dictionary

You need a good English dictionary. An English dictionary is the most important thing that you will need when learning English. Successful English learners use their dictionaries all the time — that's how they learn to use new words.
How to buy a good English dictionary?
1. It has to be an English-English dictionary.
2. It must give phonetic transcriptions (pronunciations) for every word.
3. It must give example sentences for every word.
4. It should be a software dictionary.
Example sentences: more important than definitions: If you want to improve your speaking/writing ability, read the example sentences in your dictionary. They show you how to use a word and they program your brain with correct English.

Reviews of good dictionaries

Collins COBUILD Advanced Learner's English Dictionary (4th/5th/6th edition) — It's very easy to learn English from this dictionary, because it is full of real sentences from English books, newspapers, recordings, etc. Even the definitions are full sentences. Includes the CD-ROM (see below).
Collins COBUILD Advanced Learner's English Dictionary CD-ROM (5th edition) — included with the paper version. Same friendly definitions and great example sentences, but you can look them up much faster than in the huge book. The CD also has a Wordbank with even more example sentences. Highly recommended.
Random House Webster's Unabridged Dictionary CD-ROM — a very comprehensive dictionary of American English with phonetic transcriptions and American recordings. Complements the Collins COBUILD Advanced Learner's CD-ROM nicely.
Cambridge English Pronouncing Dictionary (16th edition) with CD-ROM — a good source of pronunciations that are not included in general English dictionaries, e.g. pronunciations of people's names, geographical names, company names, and inflected forms of words. The CD version has great British recordings, pronunciation exercises, and a Sound Search feature.