Without a doubt, OpenOffice.
It's free, cross platform and (mostly) MS Office
compatible.
NeoOffice is an OS X version of OpenOffice. Since OpenOffice (for the Mac) is Unix based, and runs under the X11 interface, it lacks some of the "Mac" feel and the menus are a bit unusual (at least as of the last time I used it).
NeoOffice just takes OpenOffice and places it in a format that runs properly within the Mac OS X interface. So, it behaves, looks, feels, etc. Just like any other Mac program.
I think both are great. But, I think NeoOffice is probably going to be much more comfortable for novice users.
NeoOffice has finally got their 2.0 versions coming out. So, they are now incorporating all the features of OpenOffice 2.0.
Anyway, if you haven't checked it out for a while, you should take a quick look and see what you think.
I'm not telling you that you should change. Some people like the Unix and X-Windows feel. But, for those that don't, there's NeoOffice 🙂