I found some interesting meta tags on Eric Vitiello's site called Perceive Designs which allows you to put even more semantic information to your weblog. Examples are some geographic information, dmoz IDs and autodiscovery of blogroll OPML files.
I've included them into my weblog, so have a look at the quite growing header of the HTML source, if you're interested.
Here are some useful links, which helped me to include the right meta tags:
* Metadata Help on Syndic8
* TGN - The Getty Thesarus
* DMOZ - the Open Directory
* GeoTags - GeoTags
* List of Meta Tag values at Bauser.com
I'd encourage everyone to put all this valuable information into their own websites. That's the foundation to build tools which make use of this information!