The following list is of formats and standards that are broadly accepted within the DP milieu along with descriptions of how they operate at a technical level along with links to specification documents, machine-readable schema, test suites, open-source implementations, BNF grammars and documentation for those writing tools and parsers.
OPML
OPML is an XML-based standard for transmitting outlines, including distributed directories, notes and subscription lists. There are two versions - OPML 1.0 and 2.0. They are broadly similar, with the exception that OPML 2.0 codifies how OPML may be extended and specifies a new native type attribute value called 'include'. The primary use case for OPML is for sharing subscription lists between RSS/Atom feed reader applications. In these applications, often the outline structure is 'flattened', and off-standard implementations of OPML are commonplace.
Specification: 1.0, 2.0
Unofficial RELAX NG schema and test suite
Validator
Extended discussion of parsing OPML/converting to (X)HTML/XOXO on microformats.org
Parsing OPML is relatively easy - but one needs to be careful of a few things. Firstly, as all text is stored within the text attribute, one has to encode all special characters. If you are simply using OPML for moving subscription lists around, it is often easier to ignore the provided text attribute and simply grab the feed to find out the title.
Friend of a Friend (FOAF)
FOAF is an RDF vocabulary, specified in the Web Ontology Language (OWL) that allows you to represent a number of commonplace entities and relationships inside social networks including people, agents, groups and documents. Parsing is possible using a variety of libraries in different languages.
Specification and OWL (if you are implementing a tool based on FOAF, it is worth reading the comments in the OWL file as it specifies things in English which are not specifiable in OWL).
http://opensocialdirectory.org/wiki/List_of_FOAF_Resources
http://opensocialdirectory.org/wiki/List_of_FOAF_Containers
http://esw.w3.org/topic/FoafSites
