This is a list of previous efforts at location / geographic ("Geo") data formats and protocols, as background research for developing a location format.
vCard & hCard
vCard (RFC2426) and ["hCard"] includes 'adr', a way to markup addresses. There is also a field called 'geo' for lat/long.
Comma seperated list of lat/long
<meta name="ICBM" content="XXX.XXXXX, XXX.XXXXX">
- Flickr GeoTagging A Greasemonkey script for adding lat and long tags to flick pictures. He uses Google Maps to get the lat and long. Users have to enter search terms (address, etc) and then select a point on the google map. Photos are tagged with "geo:lat=xx.xxxx", "geo:lon=xx.xxxx" and "geotagged" and aggregated at 
- A Flickr Group related to the topic
- A visual walkthrough of the technique.
- As of today (2005-06-01), 14k photos have been geotagged on Flickr.
Several people have tried putting geographic information into RSS.
- This site uses icbm coordinates and has plugins for several blogging packages.
- BlogMapper - appears to be adding a namespace to RSS.
- BlogMap - picks up RSS extension and meta tag values
Both of these systems are freely available sources of address to lat/long mappings (US only). I think they're based on US Census data, so they might be up to date.
Plazes is a system that does geo-info based on the network access point that the user's computer is connected to. They use tagging to identify locations.
- Geosemantics Interest Group
- Open GeoSpatial Consortium
- GeoOnion - a SW vocuabulary for relating items by distance from each other.