Online Persian Lexicons

Farsi 123:
Similar to or based on Aryanpour apparently. Description found online: "
A unique online Persian-English/English-Persian dictionary that contextualizes words with example sentences, connotations, as well as references to words in Wikipedia,Google Images, and more. The most up-to-date dictionary available for free."

This is a particularly useful dictionary site, since it will provide results from the Moʿin dictionary, and Dehkhodā, and a Persian-English dictionary.

Useful primarily for translating Persian-->English, especially classical texts. You can input queries in Persian script, or in transliteration (you'll have to get to know their transliteration conventions). A variety of parameters available. Also important: Steingass has a huge number of Arabic words indexed, as they were used in medieval Persian literature.

This is a particularly thorough, complete Persian-Persian dictionary. A lot of the entries include examples of the words in use, mostly quoting from classical poems. You can also search Dehkhoda for phrasal verbs (e.g. bar-nehādan) and it will give you the proper definition. This dictionary even corrects for common spelling errors in search queries (e.g. if you wrote a sin where it should have been a ṣād). An unbelievably awesome dictionary.

Arabic Almanac:
Technically an Arabic dictionary site, but it can often be helpful for classical Persian. (Seriously... you might be surprised!) Enter the root letters of the Arabic word you want, and this site will show you the relevant pages from Wehr, Hava, and Lane. (You can choose which of these dictionaries to show/hide.)

A basic (and unfortunately titled?) Persian<-->English dictionary. Sometimes helpful, particularly for more modern usage. Also of note: This dictionary site has a pretty good iPhone version!

Google Translate:
You never know. This is occasionally helpful, particularly for modern and technical terms.

If you ever need to figure out more technical names for things in Persian, try going to the relevant page in English, and then switch to the Persian for the same entry, if it exists. (For example, this might tell you that the modern idiomatic phrase for "family planning" in Persian is "tanẓim-e khānvādeh".)

Check the second item under "Books". You can download all three volumes (in PDF) of Doerfer's classic dictionary of Mongol and Turkic words that entered medieval Persian usage. Pretty awesome!

Another classic dictionary provided online through DSAL @ U. Chicago. This one is good for Urdu, classical Hindi, Perso-Arabic, Devanagari, and beyond? into English.
Here you can download Farhang-e Eṣṭelāḥāt-e Ruz, a great Persian-Arabic dictionary that is now in wide use in Iran. (h/t Maryam Sabbaghi)
Richardson's 1852 dictionary of Persian, Arabic and English. Probably more fun than useful at this point, but who knows?