How to open Mac Keynote presentations on Ubuntu?

Create an iCloud account (https://www.icloud.com). Its free. Upload the keynote presentation. Once uploaded, you can click on the file and "download a copy" the file as either a PDF, PPT. or a key file.

The best option I found so far:

• unzip the file (this is a regular zip archive)
• take a look at /QuickLook/ directory, there is a Preview.pdf with the basic contents of the presentation.

There is an online conversion service at http://www.zamzar.com/ (assuming that your file does not contain confidential information ...)