The state of bilingual Persian dictionaries leaves, in the authors humble opinion, some room for improvement - especially when it comes to what is accessible online. Many are either limited in their coverage, or integrate auto-generated content, causing quality issues. Still, there are some that can be recommended.
WiktionaryHas a fairly large number of Farsi words.
The best English/Farsi dictionary I have seen so far is actually the English-language Wiktionary, which sports a significant number of entries for Persian words, and often comes with the usual goodies you'll find in a Wiktionary: inflection tables, pronouncations, etymology.
The English Wiktionary of course also contains entries for English, and so you'll also be able to lookup an English word, and often, you'll find a link to the Persian translation. It's not as user-friendly as a dedicated two-language dictionary, as you'll be browsing through a list of translation into dozen of languages, but it will work.
VajjeA meta dictionary combining multiple Persian dictionaries, of different languages.
Collects a number of different dictionaries, some of them very specialised, such as 6155 words and abbreviations about chemistry. It's not just English either, but includes Persian-Arabic, Persian-Italian and many more, including mono-lingual Persian-only.
SteingassClassic Persian->English dictionary from 1891, with transliterations and mobile apps.
Francis Joseph Steingass published A Comprehensive Persian-English dictionary in 1891. It is Persian->English only, and does feature transliterations.
Content-wise it no doubt shows it's age, and the way it is presented - full pages from the original dictionary rather than individual word entries - is a bit strange. But it will be useful resources for deeper study of older texts in particular.
Another classic dictionary published by the same university is the Sulayman Hayyim.
FreeDict Farsi/GermanDictionary based on community-submissions
This dictionary has an interesting backstory. The guy who runs it in his free time does not actually speak a word of Persian. There is an article about him and the dictionary in the German newspaper TAZ.
High-quality Persian-German and German-Persian dictionary.