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.