Links to Google Books and Archive.org for online versions of many English-Arabic dictionaries.
This is a sample of online English-Arabic dictionaries. Search Google Books or Internet Archive for more, and for multiple editions of some of the books below. All Google links below used to give the entire book as a free download, but many are now only available as "preview".
Wehr (below) is modern, and was the standard Arabic dictionary when I was in university. [J.W.]