Useful Arabic-English Dictionaries Online:
These are the fastest, detailed and most user-friendly dictionaries I’ve found online.
Extremely Useful Dictionaries / Thesaurus in PDF (Adobe Reader):
Near Synonyms – Nouman Ali Khan: PDF
Dictionary of the Holy Qur’an [Ahmadi (non-Muslim) created Dictionary so be cautious.
Also check here for more Useful Arabic-English dictionaries available Online.
We are working on a unique project; the Bayyinah dictionary, the first detailed Classical Arabic -> English Dictionary Online (a Collection of Words & meanings from the Tafaseer given by Ustadhs’ Nouman Ali Khan and Abdul Nasir Jangda), available soon inshaa’ Allah.