You can store any files at all, as long as they fit.
One tip: you can easily share any file with others by putting it in the "sites" folder. Then you can just tell your colleague/friend/whomever the url as follows:
(You've already dropped the file "report.xls" directly into the "sites" folder on your iDisk.)
The url is "homepage.mac.com/yourname/report.xls
" and it can be accessed cross-platform using any web browser.
The advantage to doing it this way is that only someone who knows or can guess the name of the file can access it that way, giving you some limited security.