Im a big fan of the Six Revisions website, and I suggest you add it to your bookmarks. One of the latest articles I was reading on their was

5 Web Files That Will Improve Your Website …. cover robots.txt, favicon.ico, sitemap.xml, dublin.rdf and opensearch.xml. Their purposes range from helping search engines index your site accurately, to acting as usability and interoperability aids.

It is well worth reading the full article, which can be found here