© Apple, Inc. All rights reserved.
$79.99 (included in the iWork bundle)
Numbers from Apple is a very slick spreadsheet program. I liked how you had distinct tables on sheets, but this combined with the fact that I had to export it to Excel made it less than ideal anyway.
Unlike Pages, there was no easy script to export my Numbers files, which meant that I had to open each one and export them as Excel. Not nice.
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.
After exporting Numbers documents I was able to open them fine in OpenOffice. The only difference is that any other spreadsheet software will open each table as its own sheet.
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.
That said- because Numbers tries to change the way you work with spreadsheets- it will not look as you hope it will.