You can use TextEdit (included with OS X), it has support for txt, rtf, rtfd (with attachments, e.g. images), odt (OpenDocument from OpenOffice), and some support for doc and docx.
There's Microsoft Office for Mac, of course, with full support for doc and docx (macros might not work though, check the MS documentation). Word also supports rtf of course.
OpenOffice / LibreOffice / NeoOffice support doc/docx quite well too, AFAIK, but their native format is odt. They also support rtf.
Apple sells iWork with Pages, whose native format is .pagesand incompatible with everything else.
There are a few other word processors with fewer users, like Mariner Write, but I don't know about their file format support.