Well, I'm one of the fortunate ones to have the school give me Office 2004 for free, so I take that. As for others who don't, there is the option of Parallels, but there is also iWork (which contains Pages (Word replacement), Numbers (Excel replacement), and Keynote (PowerPoint replacement). It's $71 for students, but I have heard that some people are a little lukewarm toward Numbers. I don't know how that would affect most students.

Of course, if you don't want either, there's OpenOffice.org (although on Macs, I would go with the Aqua-native version NeoOffice rather than run OpenOffice.org through X11).