OpenOffice is good too. A lot of people here prefer NeoOffice since it doesn't require X11, but I like the X11 better as it is more up-to-date and sports a better interface. I personally use Pages, but I have OpenOffice around for the occasional spreadsheet.
Edit: Wow that was a lot of replies fast! Anyways I would second LaTex (in the form of TexShop) if you are into that kind of thing... it is VERY valuable to learn and extremely powerful... if you are a physical scientist you essentially need to know how to typeset in LaTex.