OpenOffice has been ported to a number of different platforms, including:
The Mac version requires X11 installed – if you are not sure about this or need a step by step instruction manual on how to go about this, there is one on the site for you to download (PDF format) and follow.
Links for OpenOffice for Mac:
I”ll add more links as I find them.
Update: Try NeoOffice – now that I have a Mac, I’m finding that NeoOffice works really well, with all the advantages of OpenOffice but tweaked for Macs. NeoOffice – A free software port of OpenOffice.org to the MacOS X platform.