Learning foreign languages and other subjects in self-study

Arabic

Arabic is spoken in the Middle East, North Africa and other countries, for example: Algeria, Bahrain, Chad, Comores islands, Egypt, Eritrea, Iraq, Israel, Jordan, Kuwait, Lebanon, Libya, Morocco, Oman, Westbank & Gaza Strip, Qatar, Saudi Arabia, Somalia, Sudan, Syria, Tanzania, Tunisia, United Arab Emirates, Yemen.
Approximately 215 million people speak Arabic as their native language, besides it's also the language of Islam. Arabic is part of the Semitic language family, so there are similarities to e. g. Hebrew and Amharic.

What to expect when studying Arabic: English speakers will find that Arabic is not nearly as standardised as English. Students of Arabic typically start by learning a standard high form of Arabic, which is used by educated Arabs from various countries to communicate with each other, but the language actually spoken natively in those countries varies a lot, so much that people from e. g. Tunisia and Syria can't understand each other in conversation if they haven't learned standard Arabic.

Advice for learning Arabic: The script is not that easy to master. On the one hand it is an alphabet, so you don't have to memorise many different characters, on the other hand the vowels are not usually written, so you have to know the word in order to be able to read it correctly. I am planning to write a course teaching the Arabic script in a similarly easy and enjoyable way as this course for Korean. If you're interested or you'd like to help, please contact me.
When starting to learn Arabic, it's probably better to get a textbook that gradually introduces you to non-vowelled Arabic. And because of the many sounds that don't occur in English, you will also definitely need a CD or sound files to go with the course. Get advice valid for learning any language here.

Learn Arabic online and for free:

Online courses:
Course: Babel Arabic
Arabic course in French
Learn Syrian Arabic, Audio
Course: Fun with Arabic
Creative relaxation exercise: Arabic
Advanced sustainment and enhancement course
FSI Arabic courses
Tunisian Arabic lessons for Peace Corps

Arabic online dictionary

Professional (paid) translations from/to Arabic

Information
Information in German

Arabic alphabet

Learn to write Arabic

Grammar
PDF file on Arabic conjugation

Exercises

Vocabulary with Iraqi Arabic pronunciation

Test your knowledge of Arabic
Another test

Lots of further resources

Good materials for studying Arabic offline, from Amazon:
Arabic electronic dictionaries, translators & software
Standard Arabic: elementary-intermediate course
Teach yourself Arabic
Learn Arabic the fun and easy way
Arabic-English dictionary
Read & Write Arabic
More from Amazon...

Misc resources, particularly for intermediate and advanced students of Arabic:
Tongue twisters in Arabic
Learn how to curse in Arabic
Arabic music
Arabic calligraphy
Bible in Arabic
Arabic jokes