Been thinking about non-OSM tags recently while playing with flickr.

Some thoughts about amenity=post_box turned into an idea to tag flickr with
ukpostbox:id=REF_NUM see http://blog.elwell.org.uk/2010/07/m-m-m-metadata.html

following on from this, as I'm interested in AIS (Ship identification) I also propose to tag ship pictures in flickr with

ship:mmsi=..... or ship:imo=.....
(ship:name=... is to prone to duplicates, just look at http://www.itu.int/online/mms/mars/ship_search.sh and say search with ship name of 'victory'