When Spotify launched its API back in April with a great song and dance, much was made of the possibilities it could bring. Except that when it finally emerged, it turns out that you couldn't do very much with it. Many were disappointed, and Spotify's attentions turned to its mobile clients instead.
Now that the iPhone and Android apps are out in the wild, Spotify seems to have begun work on its APIs again. It's just released a Metadata API that'll allow programmers to query Spotify's vast library of music - just like searching in the client - but returning XML data that can then be output into a program or website.
It could be used for all kinds of applications, including a web interface for playing Spotify like the one run by We7, where you could search for artists or songs on a site and then get a Spotify link back. You could also build a script that automatically turns Spotify links on websites into artist and track titles.
There's a couple of limits, though - you can't query the API more than ten times per second, per IP address. And the terms and conditions have some pretty harsh language about what you call the tool you develop - it can't have the word "Spotify" in it, for starters.
You can grab the Metadata API from Spotify and start tinkering today. Let us know what you come up with.