Al-Waraq
Extremely well organised Arabic web site providing information in the following categories: Adab, History, Ansab, Geography and Travelogues, Hadith, Tarajim, Philosophy and Logic, 'Ulum al-Quran, 'Ulum al-Lugha, 'Aqida, Medicine, Interpretation of Dreams and Miscellaneous sciences.

Among its dictionaries is the most comprehensive Arabic-Arabic Lisan al-'Arab by Ibn Manzur.

 
 
 
Nida' Al-Iman
A vast online Arabic book library, including Kashf al-Khafa and the important Arabic-Arabic dictionary Qamus al-Muhit by Fayruzabadi.
 
 
Al-Islam
This site has a large collection of classical Arabic Islamic texts and its Hadith and Tafseer library are particularly rich containing nearly all the famous pieces of work in the particular filed.
 
 
 
Al-Maktabah
Dar al-Kutub al-'ILmiyah, a major publisher of Arabic books in Beirut. In addition to their own books, they can supply customers with books published by other publishers.