Flickr "tags" are user-created keywords that describe their photos. If two or more users hit on (or agree upon) the same tags, all photos with a common tag are grouped together. That's pretty cool — a kind of Wiki-style serindipitous metadata thing. What's cooler is that every tag automatically gets an RSS feed, so that you can watch all the photos tagged with "cuba" or "outdoor" or "red" in your RSS reader, getting alerts every time a new one comes along. Here's the 100 most popular tags in Flickr — click each for a link to its RSS feeds (bigger words in the list represent more-popular tags). (Disclosure: I'm an advisor to Ludicorp, the company that makes Flickr).
(via Gomi No Sensei)
Update: Joshua notes, "the tagging system in flickr was inherited from del.icio.us"
