At Last.fm we launched a public version of our playground. One of the motivations has been to be able to gather more user feedback on some of the ideas we have. In particular, to find out if ideas we have are sticky or if they are just nice ideas. Eventually, all sticky ideas will (hopefully) find their way into the main site.
I'm very excited that we now have this platform. So far we've been running most of our experiments and evaluations in the background but now we can directly interact with users and expose possibilities we have even if there is no straight forward way to integrate them into the main site.
Special thanks to Klaas Bosteels who's done some amazing work during his 3 month internship at Last.fm building up playground. Btw, if you know someone who might be a good fit for an internship at Last.fm, let me know! (Java/Hadoop, PHP, Python, and being able to work efficiently on Linux systems is more or less a requirement.)
Btw, from a MIR research perspective I believe the multi tag search is the most interesting. Primarily because it indicates how much potential tags have.