It adds active publishing to the passive publishing that I'm doing from aggregated play data from multiple music services. So, when you are thinking of a song you just type it in, and it looks for a free-range MP3 version of that song, shortens the URL, and formats it so that playTwitter knows how to deal with it. This is similar to what I talked about before except I have automated the formating, finding and posting to Twitter to make it more user friendly.
I've also added a field that let's users create collaborative playlists using Twitter hashtags (see previous post). This lets any/all people to add songs to a particular playlist. For example, see the "bestof2009" playlist. By default anyone can add songs to this playlist just by using the same name/hashtag. Well, that's great, but what if you don't want other people's junk in your list? Just use the "advanced search" features of Twitter Search to filter it down. For example, here is the list with just my contributions.
Just like on any other webpage with MP3 links, the playTwitter bookmarklet lets you "play the page". So, filter it however you want, then play it.