I really appreciate your discussion because I've been looking for a reliable dictionary lately, and I just tried www.glosbe.com to translate English to Arabic, it's really useful and different than any other dictionary I used before. Thank you Alan :)

@mumtaz: Thanks for your feedback - perhaps for French, Spanish, Japanese, etc. Glosbe might be less impressive because there are so many other options out there (eg. for English-speaking learners of Chinese, www.nciku.com is way better than Glosbe). But I'm not surprised it's helped you for Arabic, as there are surely fewer English-Arabic/Arabic-English resources as in other languages (and no doubt far, far fewer for English-speaking learners of Arabic than for Arabic-speaking learners of English). I discovered Glosbe because there's almost nothing online for Farsi learners like me.

@Jmat: I agree, Oxford is a wonderful monolingual dictionary. It's the one I use myself as a native English speaker whenever I need to look something up in English. There's a lot of debate over whether learners should use bilingual or monolingual dictionaries - I believe a mix is preferable up until the higher levels. From pre-intermediate upwards learners should slowly begin to use both, hopefully switching to 100% monolingual by upper-int/advanced. However nobody disagrees that using a monolingual dictionary is an important step all learners need to take eventually, so your recommendation is very useful.

No idea how good the paying bilingual option is, but I've seen paper bilingual Oxford dictionaries and unsurprisingly they gave much less depth than the monolingual version. Would be interesting to hear from someone who's used the bilingual online option.

The Oxford Dictionary already does a lot of what you mentioned: it gives lots of examples and will tell you if a word is regional, or if it's formal or colloquial. It's also widely considered to be the most 'standard' dictionary of the English language: http://www.oxforddictionaries.com/

Unfortunately I only tge English-English dictionary is free. If you want to translate from, say, German, you have to buy a subscription.

Thanks for the link Alan. I use Linguee as my go-to translator. The site allows you to search by words or phrases and gives examples of how they are used in context. 

An even better option for advanced users is the concordander (google that word, there are many examples to choose from).

A concordancer will separate English into "spoken", "written", "academic", "UK", "US", and many more categories. However they're generally only available in English-only, so may be a little difficult to use for lower level learners. Give it a try, though!