Twitter App: TwitterLeague
In August 2008 I set myself the task of creating a web app in 7 days and in that time I have created TwitterLeague, the site where you can create a league of any Twitter users that is based on Twitter follower counts. For instance check out this link to see the Diggnation League. WordPress.com is rubbish at being able to embed anything so instead I have an image of the Diggnation League for you to see.
The site is written in PHP and uses the Twitter API to check users credentials and to grab user follower counts and other details. The latter of those 2 calls is used both on-the-fly when a user is added to a league but also in a cron job run every hour.
Each league has some JS embedding code and also an XML feed containing all the data in the table, so you can create your own look and feel.