Editing Japanese Text Documents
Once you have enabled Japanese input on your Mac (as described on the first page of this site), you can write Japanese documents with compatible text editing software. Here a few options that work well for writing in Japanese.
TextEdit. You can start off with this simple but powerful text editor that comes with OS X. TextEdit has the very useful ability to open and save files in a range of different encodings.
Apple's Pages has good support for "ruby" or furigana text and some special text formatting options used only in Japanese. I've used it to combine Russian, Greek, Chinese, Japanese, Arabic, and Hebrew text in a single document when creating posters for our Comparative Literature program.
Microsoft Word includes a full lineup of features for working in Japanese, including furigana, vertical text, and special search options. To activate these features in Word or other Office programs, you need to use a program called the Microsoft Language Register. It should be in the Microsoft Office folder under "Additional Tools"; just open the application and it will give you instructions.
iText Express from LightWay is a free Japanese word processor with English menus and documentation. It combines the simplicity of Apple's TextEdit with some advanced Japanese support, including vertical text and Japanese manuscript (genkô yôshi) layout. The developer also produces more powerful paid versions of the program with features like Ruby/Furigana. iText Express and iText Pro are available on the App Store.
I don't do graphic design work, so I mostly make do with the Japanese fonts distributed with OS X. For those who want something more, here is a starting point:
Wazu has an excellent list of free unicode fonts available on the web, including links to many Japanese fonts you can download. The site is in English.
Hakushu Shotai also has some interesting Japanese fonts you can download free. The site is in Japanese, and the free fonts come with some license restrictions, and generally include only a subset of the most common kanji.