[iBook] alternative to powerpoint?

[J Patrick Draine]

My father-in-law is staying with us for the holidays, and is verging on 
taking the plunge, and getting an iBook (after marveling at our family 
passing our iBook around the living room as we do many email and web 
activities wirelessly).
So he's asking me exactly how much it will cost him, etc. After 
asserting that he won't need to buy any software beyond the bundle that 
comes with the iBook, I am having some minor reconsideration on one 
point. Should he buy Office X so that if he downloads a Powerpoint file 
he won't be flipping out when he can't open it? Or is there some 
alternate software that can open up such a file?
Thanks,
Patrick Draine