Online Arabic Lexicons
Probably the most complete Arabic-Arabic online dictionary. You can choose any combination of the following classic dictionaries: Lisān al-ʿArab; Maqāyīs al-Lugha; al-Ṣaḥḥāḥ fi'l-Lugha; al-Qāmūs al-Muḥīṭ; al-ʿUbāb al-Zākhir.

Arabic Almanac:
Enter the root letters of the word you want, and it will load the relevant pages from any combination of Wehr, Hava, and Lane. Holy crud!

Google Translate:
To be used with caution, of course...

Google Ejabat:
If you ever have an Arabic linguistic or grammatical question, or really any question, try asking it here! A pretty amazing online community for Arabic speakers.

If you ever need to figure out more technical names for things in Arabic, try going to the relevant page in English, and then switch to the Arabic for the same entry, if it exists. (For example, this might tell you that "hedge fund" is "maḥfaẓa wiqā'iyya" in MSA.)
Richardson's 1852 dictionary of Persian, Arabic and English. Probably more fun than useful at this point, but who knows?
Dictionary of the Holy Qur'ān by Abdul Mannan Omar.