I remember using machine tags Last.fm back, back in the day (late 00s, maybe even 2010). That was still the time of the mashup/remixed web pages, and APIs were more open and you could do very fun stuff crossing Google Maps, Facebook, Craigslist and whatever.

Then, Last.fm was a good site to keep track of the events (concerts/festivals) you were planning to go or you had visited.

Every event had an ID (let's say, 3792998), and you could add photos to Flickr with a machine tag with certain prefix (in this case, the tag would be lastfm:event=3792998). So, when you visited an event on Last.fm, it'd query the Flickr API for photos tagged with that machine tag, and show photos from that concert.

It was cool, but I think Last.fm removed that feature at least 10 years ago...

Here you can still list Flickr photos tagged like that:

https://flickr.com/photos/tags/lastfm:event=*

Edit: Oh, I didn't remember that Flickr would also link back to the Last.fm event. The Internet was quite a different thing back then. Flickr blog post about the feature: https://code.flickr.net/2008/08/28/machine-tags-lastfm-and-r...