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$79.99 (included in the iWork bundle)
Keynote is the cornerstone of the iWork bundle. It was built for Steve Jobs to do his keynotes with and it is a very slick app. But like its fellows in iWork there is on interoperability. You can only export to the ‘common’ Microsoft Office formats to use it with something else.
A big player in the open source community, OpenOffice is included in every Linux distribution I know of. It's integrated fairly well and has an extensive feature-set.
I really don't do presentations often but I believe a Powerpoint export would work fine. Of course it wouldn't support their cool transitions, etc.
A non-free alternative is ThinkFree office which happened to be giving away free copies of it's Mobile (notebook) version when I was switching. It's a Java based app, but you'd never know it. It's very nice. A little odd at times. But in some cases there may be features you need that you can't find in OpenOffice.