Keynote
http://www.apple.com/iwork/keynote/
© Apple, Inc. All rights reserved.
$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.
Migration Score
BAD
Migration Options
OpenOffice
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.
ThinkFree Mobile
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.