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.
A Frequency Dictionary of PersianExcellent Frequency Dictionary
This is the only frequency dictionary of Persian I have seen so far, and it is very good. In contrast to most word frequency lists you can find online for the language, which just split a sentence by space characters, the authors here went through the trouble to process their corpus with a morphological analyzer. So you won't find for examle ast in the list - rather, they correctly count it as being in fact the third person singular of the verb budan (to be).
There are still some disatisfying limitions. For example, because an automated process cannot tell if the word کرد in their corpus is a form of the word kardan, or rather the poper noun Kurd, they have to list at as its own entry, therefore counting neither kardan nor Kurd correctly.
They explain their methodology in great detail in the first chapter of the book, with reference to a number of papers, which is great to nerd out on, if you are into this kind of thing.
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.
Dehghani Learner's DictionarySmartly designed for learners
The full title is Persian-English English-Persian Learner's Dictionary by Yavar Dehghani.
This is the best dictionary for learners I have seen, because of some very sensible decisions: First, to include the English transliteration (which will give you the pronouncation). More importantly, the entries are listed in alphabetical order according to the transliteration. That makes looking up words very fast.
A small critique would be the lack of usage examples. It's really just Persian/English word pairs.
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.
AbadisMonolingual, with a Persian-only UI
A very nicely designed website, sourcing a number of dictionaries, both monolingual, and bilingual. Unfortunatley, the UI itself is in Farsi only, so it is not that accessible for learners. Also, you'll run into the frequently encountered synonym mess here as well (which I gather comes from generating a reverse-dictionary). You can also search for English words.
The farsi.school DictionaryA work-in-progress Persian/English dictionary
This dictionary is based on the word list we use at farsi.school - it's very much a work in progress. It mostly does not fullfill the actual purpose of a dictionary - it has very few English definitions (we're working on it). However, you may find it useful to lookup the transliteration of a word or hear a growing list of pronunciations. You'll also find example sentences there in the future.