Flickr Machine Tags. A new feature for API developers that lets them stuff arbritrary namespaced key/value pairs in to tags and query them using the API. Even without range queries, this will enable a ton of exciting new third party developments.
Sign in with 
OpenID
Very useful, but I think there needs to be a registry of namespaces and tags.
Also, there's a reason XML namespaces map to URI cnames; I can pollute someone's flickr namespace with abandon right now. :-/
Aaron has quite a neat solution to that - he suggests using an xmlns: style machine tag to indicate a namespace:
dc:subject=tags
xmlns:dc=http://purl.org/dc/elements/1.1/
The API lets you do 'and' queries, so you could search for machine tags AND their fully qualified namespace. You can do wildcard queries as well so you could find xmlns:*=YOUR-NAMESPACE-URL and work from there.