From my Friday, "it's not directly search but cool" file. If you have access to the Windows Live Messenger (WLM) beta (I don't) it's a brand new feature that reads like something I'm really looking forward to checkng out as soon as it becomes available to a larger user group which should be soon. It's a new IM feature that allows you to share a playlist and then simultaneously listen to the music the playlist contains and chat about it with friends via IM.
Microsoft is calling it Music Mix In Messenger. According to this blog post, the service will "very soon" become visible to MSN Messenger users. The post also includes the requirements to access once it becomes available. Stay tuned.