One of the complaints I hear about Twitter is there is no way to "filter" messages, specifically the ones that come via text to your phone (other than following and unfollowing people).
But what if you want to filter by the subject of the message instead of the person who is sending it?
Twitter quietly launched the first step in that process earlier this week. They call it tracking and it allows you to track the entire Twitter universe of users on your phone by keyword.
Over time I can see expanding this to include many other interesting ways to filter Twitter and I think that's a really good thing.