OPML
OPML (Outline Processor Markup Language) is a very simple XML format for sharing outlines.
What are outlines? This can be virtually everything that is like a list (probably nested), e.g.:
- Play list
- Specification
- Presentation
- Directory
- XML feed list (e.g. blogroll)
The more important thing is that people are allowed to add whatever other attributes on the <outline> element! This makes OPML very flexible. But this can also be bad. Because when someone added their own attributes, others may not know this. It is impossible to create a DTD or XML_Schema that will completely represent all possible attributes. To address this problem, some people created OML.
By the way, notice that the first applications that support OPML are made by UserLand, the creator of RSS.