After a not-so-secret limited beta, Twitter has released a new feature called Lists. It's quite simple, yet powerful at the same time. You can now create custom lists of other Twitter users, which others can follow with a single click. The topic of the list is up to you, and there are all sorts of great lists already.
Lists are like the grouping feature many twitter apps already have, but you can easily follow one or all people on it, as opposed to just reading their tweets. Plus it's managed on Twitter's servers so you can take them between different apps.
Lists will be great for social media gurus who follow thousands of people for networking purposes, but only personally know a handful of them. Likewise, it's a great way to recommend other users to your colleagues and followers, such as a "web standards" list, or "drupal wizards." You get the idea!
Twitter has already started granting API access for lists to Twitter app developers. Seesmic Desktop was the first to the table with an impressive two-day turnaround, but many more have already announced support, including Tweetie 2, TweetDeck, DestroyTwitter and Tweetings Mobile. You can check the Lists list to see which Twitter apps have it currently.