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.
I would go with NeoOffice or Abiword. They are free and open source. Also, you might be interested in some of the other great free software for OS X. Take a look here:
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.
TextEdit, my favorite 🙂 It uses the system spell check and dictionary. Has formating options, can open word, be used as a plain text editor for html, css, etc.. great little program.