OpenOffice for Mac is a free powerful office suite that provides a good replacement for Microsoft Office for Mac.
OpenOffice for Mac is a multi-platform and multilingual office suite that is compatible with all other major office suites (meaning it will open and edit Word, Powerpoint documents etc) and best of all, its absolutely free to use.
OpenOffice for Mac includes the key desktop applications, such as a word processor, spreadsheet, presentation manager, and drawing program with a user interface and feature set similar to other office suites. It may not be quite as polished as Microsoft Office but considering you get all this for nothing, you really can't complain. The six components OpenOffice for Mac comes with are Write, Calc (spreadsheets), Impress (presentations), Draw, Base (database management) and Formula (mathematical application).
There are some drawbacks to OpenOffice for Mac of course. Occasionally, you will experience formatting or printing issues when using documents that have been made in Microsoft Office. However, the main one is that since it is largely based on Java, it can be rather slow and unresponsive particularly when opening new files. This has certainly been improved in the latest releases but its still a sluggish old thing compared to Microsoft Office. The numerous settings and buttons that come with it can also be a little overwhelming too and it would be nice to see it made a little simpler and streamlined in future releases.
If you're strapped for cash or simply don't want to install Microsoft software on your Mac, OpenOffice for Mac is an excellent, if slightly imperfect alternative.