Language

• Al-bab.com - Information on studying Arabic
• Arabeyes.org - A good portal which helps you arabify Linux
• Maktoob.com - Arab Webcommunity and Web Mail
• UPenn Arab Tapes - Dialogues on Real Audio

Dictionaries

• Lessan.org - Arabic-German
• Ajeeb.com - Does not work anymore?
• Al Misbar
• VerbAcePro -  Good offline dictionary with vocalization (but no conjugations, vocab trainer etc)

Online Translation

• Al Misbar
• Cimos

Online literature and eBooks

• Divine Islam - Professional free Qur'an viewer (multi-language)
• Oneummah.net - Al-Qur'an parallel text, Arabic and English

Etymology and History

• Wikipedia.org - List of English words of Arabic origin
• Wikipedia.de - Deutscher Lexikoneintrag für Arabisch
• Etymonline.com - Etymology of modern English

Literature

• Cannon, Garland. The Arabic Contributions to the English Language:
  An Historical Dictionary. Wiesbaden: Harrassowitz Verlag, 1994 (Amazon.de)
  Erhältlich über den GVK-Plus Katalog der Unibibliotheken

Vocabulary

• Nabu - Vokabeltrainer für Krahl & Reuschel
• Chj.de - Vokabellisten und Fonts
• Lebmania.com - Spices and their translation