First of all, don't listen to people who recommend NeoOffice/OpenOffice.
These programs are horrendous. No, that does not even start to describe it. They are just so mindbogglingly awful, bloated, slow, and buggy, that if you write a document that is longer than 10 pages (or a document that has any complex formatting or tables in it), working with this document starts feeling like slow torture.
Now, for the constructive suggestions.
I can highly recommend Mellel (
http://www.redlex.com) as a replacement to Word. It is a wonderful word processor which is being actively developed and improved. Developers are very responsive, and the program is as fast and bug-free as it gets. Make sure to give a try to Nisus Writer Express too (
http://www.nisus.com). I am not using it because it does not seem to be as stable as Mellel, and because it has certain issues with typesetting subscript and superscript characters. Apple's Pages is a nice word processor, but it is somewhat underfeatured. If your needs do not extend beyond relatively simple documents (you are not writing scientific papers), it might be worth your time to check it out.
The best choice for presentation software is Apple's Keynote. Yes, even if you DO have MS Office. Keynote simply outclasses Powerpoint.
Unfortunately, there is no software that I know of that could serve as a plug-in replacement for Excel. I use Deltagraph for making scientific plots. It has more chart types than Excel, can do nonlinear fitting, etc. However, it is also expensive, and comes with a rather clunky interface - it won't be everyone's cup of tea.
I know that some people had good luck with office suite produced by Mariner, but I have not had a chance to try that. The deal breaker for me there is lack of support for all common bibliography applications.