An OPML describes a set of RSS feeds, and is a nice way of sharing a set of feeds with the world. You can use this app to generate an OPML based on a public Google Spreadsheet.
For an example, see this published spreadsheet and the resulting OPML.
Note: This is not a production-level service, it it just a tool I put together for use on my recipe blogs portal. You can deploy your own version of this or modify it for your needs by grabbing the source code from github.