Fuzzy Arabic Dictionary allows you to simply type the Arabic word you want in an over-simple form in Latin script, and it gives you back various options in Arabic script with the proper transliteration next to it. It even gives a suggestion for translation! This is a great solution for when you are away from your own computer, but you do have internet access and you really require to write something in Arabic script or proper transliteration.
Going to the website, the focus is immediately on the text box, allowing you to start typing. It responds instantaneously, with every character you type. When you hit enter, the text in the text box actually changes into the first guess in Arabic characters. You can flip to several options with up/down. So you could type something like alarabiya, hit enter, press Cmd+A/Ctrl+A, press Cmd+C/Ctrl+C, go to where you wanted to type the Arabic word, hit Cmd+V/Ctrl+V, and voilà: العربية.
It gets even better. Once you understand the syntax well, you can start typing in the over-simple form in Latin characters and with every press of the spacebar, it automatically changes the last word into Arabic script. Example: نحن في الجامعة was created by typing nhn fi aljaami’a.
The transliteration is something to be careful of. The example above gives back alʿarabīyah, whereas I would transliterate as al-ʿarabiyyah. It also gives the transliteration sign for hamza at the beginning of words, which is something we would not do (e.g. ʾādam instead of ādam).
For everyday use I advice you to set up a proper way to natively type in Arabic, as using a website like this still costs you more time and you still have a feeling of being boxed in. Nevertheless, the website is a charm to use.