A list of both Persian-English and English-Persian dictionaries, available either online or for download.

Printed Dictionaries
The new unabridged English-Persian and Persian-English dictionaries by Abbas Aryanpur Kashani
Persian-English dictionary and English-Persian dictionaries By Sulayman Hayyim
The English-Persian and Persian-English pocket dictionaries by Abbas Aryanpur Kashani; Manoochehr Aryanpur
A comprehensive Persian-English dictionary (for Classical Studies) by Francis Joseph Steingass.

Online Dictionaries
Ectaco Online Dictionaries - Online Persian-English and English-Persian dictionary, with links to downloadable ones.
Aryanpour - Online Persian-English and English-Persian dictionary
Online English - Persian Dictionary - This site offers output in several formats, including ParsiMail.
Persian Multilanguage Dictionary v1.0 (Free) - "A Free dictionary software which can be used in translation of any language to the other languages, especially Persian language to others and vice versa. The dictionary is designed to be simple, fast and flexible", authored by Dr. Amir Shakib-Manesh.
Parset - A multilingual dictionary.
Digital Dictionaries of South Asia - A Comprehensive Persian-English Dictionary (best for classical language).
Persian - English Dictionary.
Dic Beta - Persian Online Dictionary.
FarsiDic - Both English to Persian and Persian to English
DorNegar - Downloadable dictionary
Hayyim -- Digital Dictionaries of South Asia's new Persian-English dictionary
Mibo Search - Online Persian Encyclopaedic Dictionary Loghat Naameh - Online Persian Encyclopaedic Dictionary
Persian Academy - Dictionary and more
Google translate - Offers English-Persian and Persian-English translation of words, sentences, or entire webpages.