There is no way to export a calendar using Apple's Calendar app, or the icloud.com site. I was kind of surpised by the icloud site since it is possible to export Contacts there.
It is possible to export calendars using the Mac version of the Calendar app. This is a standard .ics file, which most modern calendar apps should be able to import.
I have the Workflow app, which should in principle be able to create text or csv (importabel by spreadsheet) files. Workflow is a scripting app though, which means you have to create the script. In this case it would be a pretty complex script, since it would have to iterate through calendar events and save the details one by one. It would be an intersting challenge anyway.
However, since Calendar is based on calendar standards, there is no particular reason why you couldn't use a third party (non-Apple) calendar app to access you iCloud account. That account is your's, and won't go away just because you stop using Apple products.
Alternately, you could use Google or another third party service for calendars. The Calendar app will work fine with Google, MSN Hotmail, Yahoo, and several other accounts.